Bright, popular, rebellious and creative, 21-year-old Louise Cattell had everything to live for. Then she made a fatal mistake. She took the party drug ketamine and was found dead in the bath by her flatmate. There are 125,000 other ketamine users in the UK. Louise's parents want to make sure it never happens to any of them…

When Vicky and Ross Cattell woke at the usual time on Wednesday 2 March they had no reason to think the day would not pan out just like any other. They were at their flat in Geneva, where they had been living for just over a year, and their first thought, as always, was for their children in London, Tommy, 23, and Louise, 21. Both, as far as they knew, were still safely in bed, Tommy at the family home in Belsize Park and Louise at her bachelor-girl flat in Clapton, further east.

Ross set off for work at Deloitte, the financial advisory firm, and Vicky prepared for her daily exercise routine. Everything seemed utterly normal. Then, just before his 8.30am meeting, Ross's phone rang with the news that would rip their world apart: Louise was dead, drowned in the bath after taking ketamine, the horse tranquilliser that is currently the "party drug" of choice among young people across the UK.

"When I first found out , I couldn't believe it. I thought there must have been a mistake," says Ross. "It was the most terrible shock." Vicky nods in agreement: "You have this nightmare so many times as a parent that something like this is going to happen and then you wake up and think, 'thank God, it's not true'," she says. "The problem is that now it is actually true. The worst thing that could happen has happened and there's not a thing we can do about it."

We are talking at the Cattells' house in London just a few days after their daughter's funeral. It is a lovely light-filled space, full of bright rugs and cushions, books and paintings: an elegant yet unpretentious family home. There are photographs of Louise everywhere: smiling at the camera alongside her brother, her parents and her friends; striking poses in a succession of crazy frocks, her hair a different colour and style in every shot. Beneath the stairs is an impromptu shrine to "our darling girl" complete with scented candles, flowers and yet more photographs – Louise hula-hooping in giant sunglasses and a leopard print mini-dress; Louise winking and pouting in a long black wig and matt black lipstick: Louise with lilac hair in a black corset, competing in The X Factor (she got through to the second round). Among some wilting lilies stands a solitary half-bottle of champagne, its label customised with diamanté studs. Vicky shows me the card tied to its neck. "Dearest Louise," it reads, "Something to help you on your journey".

Given what she has been through in the past few weeks, Vicky is remarkably composed. So, too, is Ross, who provides coffee and biscuits on a tray, then listens thoughtfully as his wife tells me about the last few weeks of their daughter's life. Stroking Louise's cat Pickle on her lap, Vicky recounts how Louise had returned in early February from a month in Australia visiting friends – a 21st birthday trip funded by her parents – and thrown herself into applying to art college.

Already established as a photographer and DJ, and with a track record in the fashion industry (she did a foundation course at the London College of Fashion and worked as a production assistant during several London Fashion Weeks for names such as Giles Deacon and Julien Macdonald), Louise had decided it was time to develop her talents as an artist. "She was very, very creative, always artistic as a child, but she'd never focused like this on her art before. She did amazingly well getting her portfolio together so quickly," Vicky says. "That was what we talked about during the last conversation we had – I told her I was so proud of her for getting it done in time."

It is only when she talks about the night Louise died that Vicky's voice breaks and the emotion she must be feeling spills over. "She had dropped off her portfolio to Chelsea College of Art the night before and spent the day hanging out in her flat, working in the garden. In the evening three male friends came round to supper. Her best girlfriend, who was living with Louise at the time, was also there. Apparently, they had a nice meal – Louise was a terrific cook – and they drank some wine. At some point in the evening, I'm told, Louise and some of the others took ketamine.

"The girls watched TV for a while and then Louise's friend went to bed. Louise was tired too but didn't think she'd be able to sleep, maybe because of taking the drug but also because she had things on her mind. She had given up her job [as a visual merchandiser for the fashion chain New Look] before going to Australia and she knew she had to find another one because she had bills to pay. She'd been worrying about that. So she said she was going to take a bath to help her relax. About an hour later her friend woke up and realised straight away that something was wrong. She just had a feeling. She went into the bathroom and found her. She had fallen asleep in the bath and drowned."

Horrified, Louise's friend called the emergency services who told her how to try and resuscitate her but by then it was too late.

The police called at the Cattells' home in Belsize Park at 7.30am to inform Louise's brother she had died. "It was terrible," Tommy tells me over the phone after I've seen his parents. "I can see it so clearly in my mind, it is imprinted on my brain. Me in my pyjamas, the policeman and policewoman sitting on the sofa looking awkward and tired, the disbelief I felt in my heart – I just kept thinking it could not be true." Too shocked to use the telephone, Tommy asked a family friend who was staying at the house to ring Ross and tell him what had happened.

Ross and Victoria Cattell at their home in north London after their daughter's death. Photograph: Richard Saker

Before leaving Geneva, Vicky posted a message on Louise's Facebook page. "Our darling Louise is dead," she wrote. "Please help us through this very difficult time." Within days she had been contacted by hundreds of her daughter's friends, offering and seeking condolence. On the Saturday after Louise's death they held an "open house" for everyone who had known her and around 200 people turned up throughout the day. "Obviously our own grief is terrible," says Vicky. "But for the young people involved it is also incredibly hard to bear. Her friend who found her, the poor girl, I can only imagine what she is going through. We realised we had to support them all."

Because of the scale of the response to her death, the Cattells changed their original plan for a small, private funeral and opted instead for a "big send-off" at Golders Green Crematorium. Upwards of 400 people attended the humanist service, where in place of the usual hymns and prayers there were poems written by Louise's friends and songs from her Spotify playlist, such as "Sweet Child O' Mine" by Guns N' Roses and "Just Like Heaven" by the Cure. The order of service was a fanzine-style brochure again put together by friends, full of touchingly goofy photos of Louise vogueing for the camera in outrageous, barely there clothes, mad wigs and kooky make-up, and with a heartbreakingly simple coverline – Louise 1989-2011 – that brings home the brevity of her life. The service ended with a mass singalong to the theme song from the movie Cabaret "in celebration of Louise's love of life and karaoke", which her mother thought a fitting tribute to "someone who never sat alone in her room".

Reading the many tributes on the memorial website set up by her parents, and talking to her friends and family, you get the sense that Louise Cattell was an absolute one-off. "Rest in peace you truly fantastic and much-loved beautiful person," reads one testimonial. "Louise I'll miss you for ever. You were the greatest, funniest, most amazing person I'll ever know. RIP darling. I love you," says another. And so it goes on. "She was full of energy and fun, she was always smiling and positive and you felt so happy and excited in her company," says her friend Charlie, who was a classmate at Francis Holland School in central London. "She was a true original."

Her mother recalls Louise going to school aged 12 with green hair, being sent home, only to return with a subtle shade of aubergine, which they let her keep. "I think they secretly admired her chutzpah," says Vicky. She admits she and Ross always let Louise do "exactly what she wanted to do, to a degree, and there were people, parents of her friends, who disapproved of that. But we tried things like curfews and it was hopeless. We found it worked best for the whole family when we trusted her to use her freedom wisely and she almost always repaid our trust. She got all her GCSEs and A-levels despite running club nights and doing gigs as a DJ from the age of 14. No doubt, there will be people who say all this happened because we let her be too wild but I simply don't think that is true. Dying your hair a different colour every week and staying out late does not necessarily mean you are heavily into drugs."

Louise was in no sense a habitual drug user. "I'd talked to her about drugs quite a bit," says Vicky. "She'd come across some quite serious cocaine and heroin addicts on the north London music scene and she said to me, 'Mum, I've seen what drugs can do to you and how they've messed up people's lives, and I'm never going to go there.' She may have taken the odd ecstasy tablet but she didn't even like smoking cannabis. I asked Tommy about that the other day – he said she didn't like the taste."

According to her friends, ketamine was something Louise took on an occasional basis, as many of them do too. "It's a new thing which is everywhere at the moment. People of my age are all doing it," says one of her close friends, who is 21. Louise's brother adds: "I was not surprised to hear she had taken it because it is very much part of the arty, east London scene she was involved in. It's quite socially acceptable. They all take it, it is definitely the fashionable drug of choice at the moment."

Vicky believes that while young people of Louise's age are very aware of the dangers of addiction to hard drugs such as heroin and cocaine, they are much less alert to the risks attached to occasional recreational use of ketamine. "We've been talking about this to a lot of Louise's young friends, and I think because ketamine is so widely used and available, despite being illegal, there's this idea among young people that it is somehow safe, but it is incredibly dangerous. I've found it too painful to do too much research into the drug but what I do know is you never know what is in it – and none of the people taking it seem aware of the dangers of using alcohol with it, for example. If Louise had not drunk alcohol, would she still be alive? We don't know."

Also known as Special K, Vitamin K, Super K and just K, ketamine is actually a powerful dissociative general anaesthetic originally used as a tranquilliser for horses and other large animals, which depresses the nervous system and causes a temporary loss of body sensation. It was briefly fashionable in the 1960s as a psychedelic drug but then largely disappeared from recreational use, resurfacing in Europe and north America during the 90s. It was made a class C drug five years ago, but despite this ketamine now boasts an estimated 125,000 users in the UK, and more users among young people in England and Wales than heroin and crack cocaine combined. In 2008 the British Crime Survey revealed it was the fastest growing "party drug" among 16-24 years olds, leading it to be called the "new ecstasy".

Users taking a small amount experience a euphoric rush which wears off fairly quickly. Larger amounts can cause people to "trip" for about an hour – they call it "going into a k-hole". Often they experience hallucinations and out-of-body experiences similar to those caused by taking LSD. Some report conversations with God. Because you cannot feel pain when you have taken ketamine it is possible to injure yourself quite badly and not be aware of it. Users may also become incapable of moving while under the influence, again exposing themselves to physical risk. It can cause dangerously high blood pressure if mixed with ecstasy or amphetamines. And, crucially, in relation to Louise's case, if the drug is taken with other depressant drugs such as alcohol, even a small amount can dangerously suppress breathing and heart function and can lead to unconsciousness.

Louise's brother does not believe she would have been aware of these kinds of side-effects before taking the drug. "I think she would probably have thought, everyone's taking it so it must be safe. The fact that it is only a class C drug could well have reinforced this view," he says.

David Nutt, the former chairman of the British Advisory Council on the Misuse of Drugs, suggested before he was sacked from his post in 2009 that ketamine should be upgraded from a class C drug due to the harm it can cause to users. However, Louise's parents are not convinced that a change in classification would have a great effect on the numbers of young people using it. Ross does not believe young people pay much attention to drug classifications anyway. "I think raising awareness is much more important than legislation. Young people need to be more risk-aware when it comes to ketamine," he says.

Vicky has a background in the media and PR – she has worked for the Telegraph Group and was most recently media director for the Aga Khan Fund for Economic Development – and she is planning to draw on this to start a campaign to increase awareness of how dangerous ketamine can be. "We're going to work with some of Louise's friends and some of my contacts in the film industry to put together a short film using Louise's life as a showcase, an example of a bright young life cut short, coupled with facts and figures to let young people know what is going on and to raise awareness of the dangers and risks associated with the drug," Vicky explains. "It is going to be made by young people for young people. It doesn't need to cost a lot of money. We'll make it a viral campaign, on the internet, which is where it will reach young people."

Another close friend who did not want her name mentioned claims that Louise's story has already had a profound effect on people who knew her and even on those who didn't. "This has certainly woken up a lot of people," she says. "I know a lot of people who regularly use ketamine who are stopping and thinking about what they are doing. I really want to warn people about it now." She and other friends of Louise think it is especially important to get the message about the dangers of ketamine into schools and youth clubs because the people taking it are getting younger and younger. "My friend's little sister who is 14 was caught taking it at school. Apparently loads of other girls in her year had been taking it."

As well as their campaign, Vicky and Ross are planning a memorial gathering for Louise on 21 June. This will be a simple Quaker-like ceremony where people will be able to contribute memories from the floor, after which they will launch magic lanterns over London in her memory. Next year the Cattells will stage a tribute concert to commemorate Louise's life and highlight the ketamine awareness campaign. "We want to prevent any other young person from having a horrible accident like this," says Ross. "If any good can come from this…" His voice tails off.

In the meantime, he and Vicky are faced with the unbearably painful business of clearing Louise's flat and tying up the loose ends of her young life, which had barely begun. "I've been to her flat once to collect her jewellery," says Ross. "It was quite tough. But somehow not as tough as going to the bank to close her bank account, or ringing the water company to tell them to stop sending the bills." Hardest of all was "ringing Ucas and telling them there was no point in progressing her applications to art school any further".

Louise's brother Tommy doesn't think he will ever get over her loss. "People say you grow to accept it but at the moment I just feel a really sad feeling in the pit of my stomach all the time. I'd always imagined that in 30 years' time we would still be going on holiday together, going out of each other's houses and our children would be friends. Now that's never going to happen. It makes me feel empty and alone."

www.louisecattell.com