When Claire met Seb online she thought she'd found true love, but slowly it became clear that Seb was not who he said he was. Then the other women began to get in touch. In a remarkable investigation, Tom Lamont unpicks an unsettling tale of hope, secrets and dishonesty

On a Saturday afternoon in June, Claire Travers Smith arrives in a port town in Pembrokeshire where the air is salty, the gridded streets quiet and an enormous multistorey ferry looms in the harbour, waiting to plough across the Celtic Sea to Ireland. Claire has come in from the other direction, a few hundred miles on motorway and snaking A-road from London, travelling about as far west as Wales goes. She is searching for someone she once fell in love with.

They met online two years ago, Claire tells me, almost to the day. It was through a dating website, Smooch, and she found the man compelling right away. He could spell and he knew how to be flippant and funny via browser-window text box – traits that Claire, an experienced and weary internet dater, had come to prize. "Yeah, you'll do," he wrote to her. "Now are we gonna mess about on here for weeks, gradually upgrading to texting one another, or are we just gonna meet up and go on a date?" They swapped real names, photographs, phone numbers.

Claire learned that Sebastian Pritchard-Jones was a 35-year-old teacher, a football fan who adored vintage Ford Cortinas and lived in Marylebone, west London. His photographs showed him in thick-rimmed glasses, a slender man with brown hair and a square, bearded jaw. "Nicer-looking than the men who normally messaged me back," judged Claire, 30, a TV producer who has worked on This Morning and The Weakest Link. She has long fringed hair that frames a round, pleasant face. When she and Sebastian first spoke on the phone Claire remembers prickling with attraction. He had a reedy Welsh accent. He teased her about having an unusual double-barrelled surname, like his own. He told her to call him Seb.

This was a relationship brought about by new technology but sustained by old. Their telephone conversations got longer and longer and "within a couple of weeks Seb was the first person I heard from in the morning and the last person at night". Claire revealed things to him she'd never told anyone. Seb was a good listener. He said wise things.

But he was no good at sticking to plans. When they arranged to meet for the first time in a Soho pub, he was kept late at his school and had to cancel. Then he made a mess of the rearranged date, going out to play football and ending up in bed with a torn muscle. Claire was frustrated but forgave him – organisational hopelessness didn't seem the most terrible vice in a man met online. She'd once been set up with someone who reminded her, visually, of Satan. Over the years she'd cancelled accounts with Smooch, Guardian Soulmates, Match and OK Cupid, too often disappointed. Usually she drifted back, still looking.

A third date was arranged and Seb stood her up again. Family problems, he said, buying her a gift this time to apologise. It was a bottle of perfume, a scent Claire liked. Seb handwrote a note – "Why would I buy you a bottle of perfume if I had no intention of meeting you?" – and laid it next to the bottle, taking a digital photograph of both.

Claire has looked at this image many times since; surely more times than Seb ever did. She has brought her laptop with her to Pembrokeshire, and on its hard drive, among hundreds of files relating to Seb, the picture of the perfume bottle is the most revealing. The bottle has a round, mirrored surface. It shows a reflection of someone leaning in, holding a camera, taking the photograph.

Studying the image on her laptop, zooming, squinting, Claire saw that the person she had fallen for was never slender with brown hair. No glasses, no beard. Instead this was someone blonde, double-chinned, with large, puffy arms. Claire saw a woman.

The internet is famously well arranged for deception. Most of us have encountered its cons – that distant prince, keen to share his boundless treasure in return for one or two small loans. Online dating has tricks all its own, even if most are harmless. A survey by OK Cupid in 2010 found that the majority of its 1.5 million subscribers lied about their stated height, adding an inch or two. They upped their income by about 20%.

Giveaway reflection: the bottle of perfume and the note written by Seb after he stood Claire up for the third time. Photograph:

There are sometimes more radical embellishments. In America people have been fascinated for months by the story of Manti Te'o, an American football player whose girlfriend, met online, turned out not to exist. The precise details of the case are still not clear, but a fellow player suggested Te'o had been "catfished" – a reference to the 2011 documentary Catfish, about a New York man who believed he was involved in an online relationship with a girl in Michigan. (He was wrong.) A spin-off TV series, on MTV, explored similar dating hoaxes, as did Channel 4's How To Fall in Love Online last month. More and more, it seems, love is being offered on the internet, and offered dishonestly.

"Apparently this is a whole new thing to be aware of," Claire tells me. "People who would be duplicitous for reasons that aren't financial." Soon after receiving Seb's photograph of the perfume bottle she ended the relationship. Being stood up repeatedly was one thing, but the photograph frightened her. Seb replied: "I think I'm falling in love with you." It was the last time they spoke.

Claire was writing a blog about dating at the time, and picked over the stranger elements of the affair for a series of articles. Seb liked to email photographs of himself doing dramatic things – snorkelling, paintballing, shaping his hair into a mohawk in the bath – and he also sent lots of photos of handwritten messages. One was a spider-gram, drawn carefully in red felt-tip and listing "Things I like or love". Teaching, roast dinners, football, bottled Bud, Cortinas, Claire. It almost read like a list of prompts.

Claire wrote about Seb's voice, its "smooth Welsh tones", and mentioned other details such as the four-bedroom Marylebone home he owned and a beloved niece who lived back in Wales. Seb once had a girlfriend who died of cancer and his best friend, Philip, was severely disabled. Claire published her blogs in June 2011. Before long, emails started to come.

She heard from Rachel Burton, a 44-year-old ward sister who had been in a relationship with Sebastian Pritchard-Jones, too. Rachel and Seb first met on Smooch, going on to speak to each other for hours on the phone. Rachel recognised Claire's mention of Marylebone, the beloved niece, the girlfriend who'd died of cancer, the disabled best friend. Just one thing: the Seb she'd always seen in photographs was blond, with wide-set green eyes. Ali Moores, a 32-year-old who worked in recruitment and who'd met Sebastian Pritchard-Jones through Guardian Soulmates, recognised Claire's description, too, only the Seb she'd known had angular eyebrows and spiked, caramel-coloured hair.

The emails kept arriving. "I'm the latest idiot," Danielle, a 33-year-old account manager from London, wrote. The Seb that Danielle had fallen for was dark with a square jaw, the same man that Claire had seen in photographs. When Karen (a 31-year-old who worked in the media) and Susan (35, a marketing manager from Derbyshire) contacted Claire, they said they recognised the photographs but not the name. They knew him as Harvey.

‘It was a daily occurrence to say how much we loved each other’: Ali Moores Photograph: Observer

Different names, different faces, unmistakably one person. They'd all got used to late-night drunken calls in which songs were sung down the phone, mostly football chants, though Rachel got hymns. Karen received roses; so did Susan. Ali was sent money to shop for clothes and booked into a hotel at Seb's expense. They'd all been told about the teaching job, soon to yield a promotion, and a disabled best mate, Philip.

Danielle told me: "We'd speak for five or six hours at a time. He dominated my life." Susan said: "I lost six months to it." Ali managed to get so close to Seb she would sleep with her mobile on her pillow. They had phone sex. "It was a daily occurrence to say how much we loved each other. We talked about having kids." Karen was due to be introduced to his family in Pembrokeshire, but when she got to the train station nobody came to pick her up. Danielle had been promised a trip to New York. Seb told her he'd collect her on his way to the airport.

"It was around then that my mum found Claire's blog," Danielle told me. "She was suspicious and Googled his name. I remember her texting me: 'Seb's a fraud.' Immediately you feel that what you've been feeling isn't real… As I'm telling you this, I can't believe I was so stupid. I'd watched the Catfish documentary. I even said to him: 'Please don't turn out to be a liar.'"

The most recent woman to contact Claire was Joanna Bell, a 40-year-old chef working in London. The photographs Joanna had been sent were different again (this man had sideburns and a slightly bulbous nose), but she recognised the person Claire described. Joanna had been told his name was Harry. The relationship, telephone-based, lasted for months.

Joanna was in the middle of a shift at the O2 Arena when I spoke to her, busily preparing food between gigs for Alicia Keys and Barbra Streisand. "I thought we were going to get married," she told me. "My mother thought we were going to get married." In conversation Joanna had a chef's brisk, no-nonsense way of talking. I couldn't imagine her being taken in by internet scams (that distant prince).

Every one of the women I spoke to seemed balanced and bright. "We're professional, intelligent, articulate women," Rachel pointed out. "I've got a degree in psychology," said Danielle. Karen: "It's just that your mind tells you this is the person who you want them to be." Ali: "It hurt your head and your heart so much, trying to figure out what was true and what wasn't, that it was easier just to accept it all." Joanna: "Everyone wants to be in love. You think: 'Finally, it's happening.' So of course you want to believe."

A picture of the money and note Ali Moores was sent by Seb. Photograph: Observer

Joanna was fooled at a particularly cruel time in her life. "It was just after my father had been diagnosed with cancer. He [the hoaxer] said: 'Don't worry – I'm going to look after you.' He told me to tell my father that. You know, as my dad was lying there. As they turned the machine off." After the death, Joanna and her family received a condolence card from Harry. It had a Pembrokeshire postmark.

Claire and I have come here with no definite plan as to how to find who we're looking for. We drive and then walk around the village and keep remarking on how deserted it feels. From a corner pub a woman stumbles out to argue with a man standing woozily on the curb. We search for a café, somewhere we can open up Claire's laptop and look over the information she's gathered, but all the cafés are shut.

Claire, increasingly a sort of marshal and an ad hoc counsellor for those who've been duped, has drawn up a timeline. As far as she can tell, the hoaxing began in January 2009, with Susan. Then Karen, Ali and Rachel. Claire and Danielle were "suckered" (Danielle's phrase) in 2011. Joanna's relationship lasted until summer 2012. Other women have come forward to speak to Claire, too, filling in gaps in the chronology. Despite occasional overlaps, the women were met and romanced one at a time. This hoaxer was loyal.

Everyone's relationship tended to end the same way, usually with some clinching failure to meet, or the implausible stories too thickly piled to ignore. Angry, quite threatening texts sometimes followed a break-up, and in the aftermath of her relationship with Harvey, Karen went to the police. So did Claire. Both were told that because no money had been extracted, and there had been no physical abuse, the police were powerless.

Complaints were made to dating sites, including Guardian Soulmates and Smooch, and accounts deemed suspicious were blocked. Meanwhile Susan went to a private investigator, who confirmed that the name she'd been given matched no records and was certainly fake. Ali found out that there was no Sebastian Pritchard-Jones registered to teach.

Exhaustive Googling and an appeal on social media turned up the square-jawed man from the photographs seen by Claire, Karen, Susan and Danielle. "You don't know me," Claire wrote to Craig from Milton Keynes, "but in a funny way I feel like I know you." Craig had no idea that over a period of years he'd been reimagined as a roast-loving Welshman. "I feel physically sick," he told Claire.

The man in the images seen by Ali turned out to be a construction worker called Gary who lived in Hull. As with Craig, his photos had been taken from Facebook; Gary admitted to me, when I telephoned, that he'd never been very careful with his security settings. I told him Ali had been so wrung out by her nine-month affair with Seb – Gary's face, stranger's voice – that she'd eventually relocated to Australia. Gary told me: "It's a weird feeling to think somebody was in love with you like that. I just feel really sorry for [Ali]. It's hard for me to take in, it's been a shock, but I'm not the one who's had my heart broken. There's nothing worse."

One advantage of there being so many victims, according to Claire, is that they were able to compare notes. Clues. Joanna received her condolence card from a Pembrokeshire postcode, and Karen had been given a family address there that turned out to be fake. They'd all heard talk of a nursing home where Seb (or Harry, or Harvey's) disabled best friend Philip lived – it was in the same part of Wales.

Joanna discovered that, though Seb, Harry and Harvey left no tangible traces on the internet, Philip really did exist. There was a Welsh newspaper article about him and his devotion to his local football club. She found an address for Philip's nursing home.

Digging on the internet, someone else found a suspicious account on the picture-storage site Photobucket. It was full of the purloined Facebook pictures of Craig, including the one of his mohawk in the bath. A trail was followed to Flirtbox, another dating website, where the pictures had been uploaded by someone with the username YX9AJP. An eBay account, registered to YX9AJP, had been used to buy several sim-card adapters for a mobile phone.

As the research accumulated, one name became inescapable. Karen had been sent roses, and when she contacted the florist to find out who'd placed the order she was given a name: Amy Palmer. Craig recalled meeting an Amy Palmer on Flirtbox in about 2008. They'd exchanged pictures, and for a time after that they were Facebook friends; she would have had access, he thought, to a wide selection of his pictures. Ali recalled that when she'd received money from Seb, in 2010, it was transferred from an unexpected bank account – AJ Palmer. The local newspaper article, the one about Philip, carried a quote from his carer. Amy Palmer.

An answer was presenting itself, and none of them wanted to acknowledge it. Claire studied the image of the perfume bottle again, with its incriminating reflection: "My blood ran cold."

‘Everything he seemed to offer was something a woman would look for’: (from left) Rachel Burton, Joanna Bell and Claire Travers Smith, three of the hoaxed women. Photograph: Suki Dhanda for the Observer

They all thought again about the voice they'd listened to for hours on the phone. "Unusual sounding," Claire called it. Karen had always found the laugh oddly high pitched. Danielle: "It was soft and it didn't have any gruffness." Joanna: "It sounded like a man, just an effeminate man."

"While the voice wasn't particularly deep," Rachel told me, "it never occurred to me it was anything other than male. I suppose there were characteristics. Everything he seemed to offer was something that women would look for." Claire said: "He was a teacher, so he was reliable. He owned a house, so he was solvent. And he listened." Karen told me: "You're falling in love with an ideal. And I guess a woman would know what makes a woman tick."

Claire stops, abruptly, on our walk around the Pembrokeshire town. She has seen a car with the number plate YX9AJP. We are on a quiet road, just around the corner from the nursing home that Joanna found through her research.

Phone calls have preceded our visit to Pembrokeshire, and staff at the nursing home are aware that I am a journalist, accompanying Claire on a search to find Amy Palmer. Everyone there has been polite but firm in their efforts not to help. When I first got in touch they told me Amy was employed at the home but was away on holiday; later, that she'd left her job entirely. Once I called and spoke to someone who, whispering, claimed to have lost her voice. She was adamant that Amy Palmer had not been seen for weeks.

The car with the number plate YX9AJP is outside a terraced house with drawn blinds. We press the doorbell. Claire, spirited and determined all afternoon, is suddenly shaking. She asks if I'll do the talking. In the end, not a lot of talking is required.

When Amy Palmer comes to the door of her home she is wearing a blue T-shirt and shorts. She is large, squat, with parted bleached-blonde hair. She appears to be in her late 30s. I tell her I'm a journalist and that she might have received my emails and letters requesting an interview. Having instinctively reached to shake my hand, Amy changes her mind, grasping the front door instead. She takes a long look at Claire. When I say we have come to ask some questions about Sebastian Pritchard-Jones, Amy interrupts, saying: "I don't know what you're talking about, mate." She hurriedly closes the door.

The voice is curious, deep for a woman's. Claire recognises it. And did you see her expression, I ask. Claire nods. "It made me think of a child who's been caught doing something they weren't supposed to."

When we ring the bell again, five minutes later, an older man and woman answer. Amy hovers in the hall behind. The older man, probably her father, asks what we want to speak to Amy about. I say that as it's sensitive it might be better if Claire and I spoke to Amy alone. The man turns to Amy and repeats the question. So does the older woman, who I assume is her mother. Amy says she doesn't know. Clearly distressed, she says she won't speak to us. At no point does she ask why Claire and I have come. At no point does she ask who Sebastian Pritchard-Jones is.

I hand over my phone number in case Amy should change her mind. The older man is now on the front step, clearly baffled but also protective. We leave.

Before travelling to Pembrokeshire I'd asked Claire about her motivations. Why seek out this person? "It feels like something I need to finish, for everyone, really." Joanna last heard from the man she knew as Harry as recently as January, and Claire suspects that the hoaxing is ongoing. Those suspicious sim-card adapters, seven of them, were bought on eBay only weeks ago. "She'll probably speak to someone tonight," speculates Claire as we're walking away, "with a name I don't know about yet."

Neither of us can stop thinking about the way this woman in her 30s hung behind her parents, looking "guilty as sin", as Claire puts it. "She's created this entirely fictional world, and a really good one, because it was a world that at least seven other women wanted to be part of. I just wonder if she has an idea of how much damage she's caused."

Driving out of the town, I can see that Claire is flitting between emotions. Relief at having come face to face with someone she's been tracking for months, and frustration that we weren't able to extract answers. Pity, too. Many of the women I spoke to expressed pity.

"After the anger," Karen told me, "there came a weird feeling of sympathy for whoever had done it. The time and effort they put in. It was a full-time job." Joanna, for whom the wounds were rawest, could only comprehend it in terms of somebody being wounded themselves. "I would guess this is someone who's been really badly messed over and wants to hurt other people in the same fashion. Why else, except for revenge?"

Claire has her theories. Some of the late-night conversations that Seb had with women were sexual. Claire tells me that, having visited, she could well imagine Amy's being a town, where sexual openness might be difficult. Alter egos might have seemed the only outlet.

Back in London I ask Claire to email me when she's had a chance to think over the trip. She gets in touch the next day and says that the feelings of pity have faded somewhat. "She's not accidentally deceived one stranger and fallen in love," Claire writes. "It has been a systematic campaign… I still don't quite understand what she got out of it. Only a couple of the other girls, as far as I know, allowed a sexual element into their relationships, so that can't have been the only thing. She must have started to fall for us, too." Claire goes on: "In some ways I hope that she did."

There are several made-up names in this article, beyond those three – Seb, Harry, Harvey – that caused such heartache. Claire Travers Smith, Rachel Burton, Ali Moores and Joanna Bell were willing to be named in full, but Susan, Karen and Danielle spoke to me on the condition of anonymity. After responding to my initial email – "I'd be very interested to get to the bottom of this" – Craig went quiet. Unable to secure his full co-operation, I have changed his name, as I did with Philip. Gary preferred that I not use his surname.

The name of Amy Palmer has been changed, too. She may not deserve a covering identity, another one; even so, after discussion with psychologists and with editors at the Observer it was agreed that this extensive, energetic fraud could only have been conducted by a profoundly disturbed person. When I presented the evidence gathered to an investigative psychologist, Dr Keith Ashcroft, he suggested "the temporary relief of boredom" as one of the hoaxer's motivations. He also introduced me to the psychologists' term "duping delight". Dr Ashcroft explained: "Essentially a thrill derived from having victims being intensely controlled and manipulated by carefully formulated deceptions. This is often the modus operandi of a psychopath."

A psychologist specialising in online harassment, Dr Emma Short, speculated that the "absence of other interests or close personal relationships" likely factored. Clinical forensic psychologist Mike Berry considered the case and told me: "She doesn't come across as someone who's sadistic. She's looking for the great love, and maybe it's never happened to her, so she's fantasising. I don't think she's grooming for sex. She's grooming for love."

Claire and I do not hear from Amy after visiting Pembrokeshire. A letter sent two days later goes unanswered. I want to know what it can possibly be like, repeatedly embarking on relationships with an absolute limit of duration and plausibility, everything doomed from the first message on. Claire wants to ask a much simpler question, the same question all the women want to ask: why?

Perhaps there's some sort of answer among the files on Claire's laptop. Claire has collected screen grabs of the various profiles that Seb, Harry and Harvey posted on dating websites over the years. One of them reads: "I think I'm supposed to say I love life. To be honest, though, I don't have a lot to compare it to… I think life can be complicated, and that's a good thing. I like getting older, becoming wiser, and knowing that, whatever happens, there's a way to make things good."