On one side of the Atlantic fans wrestle with an internet connection to secure tickets for Sam Smith’s just announced American tour. On the other, 30 or so wait outside the O2 Academy Leeds (show time is not for another five hours), on the off chance they will catch a glimpse of their idol. After about an hour Smith steps out to oblige the demands for autographs and selfies.

Back inside, he takes the stage for this afternoon’s soundcheck. He is wearing a long trench coat, blue suede shoes and his trademark crucifix earrings. Flanked by his backing band and three gospel singers, he belts out his set pitch perfect. During this, one of his team tells me that his debut show at New York’s Madison Square Garden has sold out in half an hour. Later, a 17,500-capacity arena in Los Angeles will sell out in 10 minutes with an additional date added hastily in its wake. Like dominoes, Toronto, San Francisco and Washington, DC, are next to fall.

In the days leading up to our interview, Smith, 22, performed on Later… with Jools Holland; won four Mobos and a Q award on the same day; announced the latest single from his number-one album In the Lonely Hour; and embarked on a UK tour that began in Glasgow the previous evening, before arriving here. His trajectory has been astonishing. Only two years ago Smith was working seven days a week in a City of London bar, cleaning the lavatories and walking home because he couldn’t afford the Tube fare.

‘The one thing I would say that I think has done it, it’s honesty. Go through any massive song and the key is being relatable' Smith says. Credit: Sam Faulkner

‘It’s quite overwhelming sometimes,’ he admits later, as we sit in his dressing room two hours before stage time. He is more casually dressed now, in a polo shirt, hoodie and jeans. ‘It’s getting to the point where I really can’t digest it. If I sat down and tried I think I would probably break down in tears or just freak out.’ He confesses it is ‘impossible to stay completely normal’, but the key, he says, is not to remain the same, rather to be ‘aware’ of the sheer absurdity of the situation, especially as, he says, ‘One day I might bring out a really shit album. It will be interesting to see who’s still your friend then.’

For now – armed with a three-million-selling debut album loaded with tales of heartache, influences that include Aretha Franklin, Amy Winehouse and Whitney Houston, and a vocal range to rival Mariah Carey – Smith is Britain’s biggest musical export since Adele Adkins. His mournful falsetto elicited heavy expectations before he had even released a single, owing to guest vocals for Disclosure (on 2012’s Latch, which he also wrote) and Naughty Boy (La La La)– with the latter earning Smith his first number one.

Once he finally resigned from his job at the bar, the recognition began, and quickly. He topped the BBC’s Sound of 2014 poll in January, which was followed by the Brits Critics’ Choice award – an industry nod that previously greased the wheels for the likes of Jessie J, Ellie Goulding and Adele. The comparison with Adele is never far away – in part thanks to the stratospheric sales, brooding lyrics and vocal talent they share, but perhaps primarily as neither conforms to the finely chiselled ideal of the modern pop star.

‘It actually makes me sad,’ Smith says. ‘There should be artists out there of all different shapes, sizes, colours and sexualities.’ Where others rely on gimmicks, pyrotechnics and salacious dance moves, for Smith and Adele it is about the voice and nothing else. ‘It’s quite frustrating actually when you do shows and you’re the only person not twerking. I get angry, like, “Why am I the only one standing there singing?” People should be doing that.’

And yet whatever Sam Smith is (or isn’t) doing is evidently working. The singles Money on My Mind and Stay with Me – both of which he co-wrote – were number ones, while the album is the highest charting debut from a British solo artist in the United States. While many of today’s musicians find themselves laser-targeted towards a certain demographic, Smith’s appeal shows no bounds.

Sam Smith, shot exclusively for the Telegraph Magazine in Leeds, November 2014 Credit: Sam Faulkner

At tonight’s gig this is further evident, teen girls jostling for floor space with veteran soul boys and countless couples. How did this happen?

‘I don’t know,’ Smith says, smiling. ‘I think it’s just me. I hope it’s just me. I make music that the builder down the road – who can’t sing at all – can sing.’

He must have a theory, though?

‘The one thing I would say that I think has done it, it’s honesty. Go through any massive song and the key is being relatable. Make a song that makes everyone go, “I have felt like that. I know how you feel.” If you strip it back, be yourself and be natural, I think people can sense that, you know.’

In the Lonely Hour is certainly a soul-barer of a record, telling tales of loss, heartbreak and unrequited love. On its release Smith made no secret that the object of his affections was a man. ‘I’ve always been an open book about it,’ he tells me. ‘This was the constant thing; I came out as a gay man at a very, very young age.’ Coming out to friends and family at 12 no doubt required courage, yet doing so to the music industry was arguably braver. ‘We all had that reservation before the record,’ Smith admits. ‘Past gay artists, things have happened to them that haven’t been great. All I can say is I’m so unbelievably surprised and happy at how far the world has come, because there have been no issues.’

Smith is yet to have a serious relationship but has come clean to the man who inspired the album (a hard but ‘very natural’ conversation, he says), and he can’t stifle a smile when he tells me that he has been dating someone of late. He says there is no time for a relationship right now, however. ‘Obviously I want to fall in love and stuff, but I don’t yearn for it every day like I used to. It sounds so cheesy but my fans have completely filled that hole.’

Smith playing cards on the tour bus with two of his bandmates, Ruben, the pianist, and Brendan, the bassist (right) Credit: Sam Faulkner

Sam Smith grew up in Great Chishill, a village in Cambridgeshire. His father, Frederick, was a house husband, and his mother, Kate Cassidy, worked as a trader for the City broker Tullett Prebon, earning £500,000 a year. Smith was an artistic child with two younger sisters, and claims to have been ‘always acting and singing’, once setting up ‘a film company’ with his neighbours and shooting DIY horror movies for local parents to watch. After hearing his eight-year-old son singing in the car, Frederick Smith arranged singing lessons with a local jazz musician, who let Sam sing anything he desired. He chose Frank Sinatra’s Come Fly with Me in his first lesson and the soundtrack to Cats in his second.

Smith is old beyond his years – a product, he believes, of being forced to hold court at his mother’s dinner parties. Often he would perform musical numbers for guests, and on other occasions simply ‘tell stories’. ‘If someone was talking about a subject, my mum would be like ,“Oh, that happened to us last week. Sam, tell everyone,’” he says. ‘At 10 years old that is quite daunting, to chat to a whole table of adults and hold your own. There’s an art to it.’ He pauses for a huge swig of water. (He used to drink whisky pre-performances, to settle his nerves. This is no longer required.) ‘If that means I’ve got an old head on my shoulders, then great. I’d never want to lose that.’

Smith had his first manager at 12 – a ‘tough’ time, he remembers. ‘I had a lot of adults around me and they could see something, but I don’t think they knew what to do with it.’ Numerous others followed, as Smith endured years of broken promises from managers who claimed they would make him famous, and false starts with record labels.

The first – wholly unwanted – taste of recognition came in 2009, when the Daily Mail reported that his mother was fired from her job for gross misconduct, amid claims she was spending company time on her son’s pop career. Cassidy filed for wrongful dismissal. Smith won’t divulge whether she won her case or not and has never discussed its outcome. Today he says that the episode only served to intensify his determination to succeed. ‘There’s a fire in me,’ he says. ‘There’s a fire that ignited in me on the day that happened.’ And it made Smith feel even more sympathetic towards his father. It was he who had served as everything from a taxi driver to personal trainer in support of Smith’s career. ‘My poor dad’s driving me around London for years and then my mum gets all the credit,’ he jokes.

An hour before the show, and Smith is on his tour bus, struggling to teach his band how to play the card game Black Maiden. ‘This is us pretending to have fun,’ he says, as he explains to the dubious band that the loser must be ‘punched in the face’. Complete with high-end sound system, flashy televisions, oversized fridge and video-game consoles, the bus is suitably lavish (though the European plug points are ‘a pain’, Smith says).

I scan the rider: champagne, beer, vodka, olives, mini cookies, Petits Filous yogurt and, rather oddly, Bisto. The mood is light and there is little sign of hierarchy. Smith dictating a Facebook update to Jack – one of three managers who help run his career – between card hands (to be posted to his 2.3 million Facebook fans) is the only real reminder it is his name up in lights tonight.

‘It’s quite frustrating actually when you do shows and you’re the only person not twerking. I get angry, like, “Why am I the only one standing there singing?” People should be doing that.’ Credit: Sam Faulkner

Smith’s sister Mabel is in town for today’s show. Her red Micra is parked behind the bus. Smith tells me it was actually his first car, the one he would drive to gigs when he moved to London four years ago. By this point he was growing frustrated at his failed attempts to get noticed. Meanwhile television shows such as The X Factor and Britain’s Got Talent were awarding fame without much heavy lifting. Was he tempted to sign up? ‘One hundred per cent,’ he says. ‘I did go on to a talent show one time but it wasn’t televised. I came third in the country or something so that was interesting.’

On the brink of shelving his ambitions in favour of pursuing a regular 19-year-old’s life, Smith finally got his break. He found his seventh manager in Elvin Smith (who is still with him), a former singer Smith had once drunkenly heckled at an Adele concert (‘This song is for someone I love,’ Elvin, an Adele support act, had said. ‘Ah, thanks!’ yelled Smith from the crowd) before bumping into him a couple of years later. Through Elvin, Smith was introduced to Jimmy Napes, who produced his album and connected him to Disclosure.

‘I’m so lucky to have met the team then because I was at the point where I was going, “I will do anything...” Not anything, but I was saying, “You can clone me into whatever you want. I just want to be a singer. I don’t want to be working in this bar any more. I will do whatever you tell me to do.” What they told me to do was just be myself.’

Two years later, and Sam Smith finds himself in the same circles as the artists he once idolised. Ed Sheeran and Jessie J have offered ‘gorgeous advice’, he says, on how to maintain his voice (rest, water and honey, if you’re interested), and he is rarely seen out without a harem of beautiful women, including his best friend Daisy Lowe, Miley Cyrus, Katy Perry and Kim Kardashian (turned on to Smith’s music by her husband, Kanye West – ‘an amazing woman’, Smith gushes). I had also read on Wikipedia that Smith is Lily and Alfie Allen’s third cousin, something I had dismissed as false (it was Wikipedia, after all), but can’t help asking about. ‘It’s true,’ he says, ‘but I know Lily Allen as much as you know Lily Allen. I have only met her three or four times but every time she sees me she goes, “Cuz!”, which is great.”

Smith's album, In the Lonely Hour, debuted at number two in the US charts, selling over 166,000 copies - the biggest selling debut by a male British artist Credit: Sam Faulkner

A drawback of fame is, of course, intrusion into one’s personal life. In the Lonely Hour paints Sam Smith as a candid individual, but he has his hang-ups. He has always struggled with his weight. He doesn’t look overweight, but at 6ft 4in he is certainly imposing. ‘I’ve got a security guard now and he’s my fitness trainer as well, so we go and work out all the time,’ he says. ‘There have been moments I have looked online and thought, “Oh God, that’s such an awful angle,” but I don’t feel any worse about my body now than I did when I was younger. It’s true what they say about TV putting weight on people. I’m not fat; I’m just not five foot something, and I don’t look like a skinny guy from a boy band.’

Five minutes before show time: Smith is dressed in a navy suit, crisp white shirt and the same blue suede shoes. They are Calvin Klein, his proudest purchase and the only item to survive all three of today’s costume changes. He recently met American Vogue’s editor-in-chief Anna Wintour, at her request, along with Sarah Jessica Parker. This was evidently a very big deal to him. ‘The top of that world has given me a nod,’ he says. ‘It’s unbelievable. I am such a big fan of Anna’s.’

Smith has only recently moved out of a modest £500-a-month flat in Oval, south London (and only because he had been burgled twice) to rent a ‘nice, safe place’ in east London, with no designs on owning somewhere until next year. His most glamorous purchase to date had come minutes before our interview in his dressing room, in fact – as he had bought a new car for his sister. It seems the Smith family’s trusty, rusty Micra is not long for this world, billowing smoke from the engine over the past few days. But even then, ‘I’m not overdoing it,’ he assures me. ‘It’s another cheap car; just not a Nissan Micra.’ His one real treat to himself backfired. Feeling lonely, he bought a blue French bulldog for ‘a ridiculous amount of money’. He sold it three days later. ‘I realised I wasn’t that lonely,’ he says, grimacing.

Smith and his five-piece backing band – all dressed impeccably in black suits – join hands and chant, a ritual before every show. Striding on to the stage and dancing playfully around each other, they don’t appear to have any nerves. The show itself is surprisingly spectacular, featuring gospel and big-band collaborations, along with cover versions of Whitney Houston’s How Will I Know and CeCe Peniston’s Finally intertwined with Money on My Mind.

‘That was amazing,’ Smith says as he comes off stage, accepting hugs and high fives from his band. ‘I honestly think that was one of the best shows we’ve done.’ There is no raucous aftershow party to host tonight – the bus is bound for Newcastle for tomorrow’s show within two hours of Smith finishing his set, so watching a DVD on the tour bus with his sister is the extent of the celebrations. En route to the bus, Smith makes a detour to thank everyone from the venue staff to the roadies dismantling the stage. Holding a box of chocolates he plans on giving one particularly dedicated fan, Smith then bids farewell to 80 or so shrieking supporters waiting behind railings outside. ‘Sam, Leeds loves you,’ one shouts. ‘Did you mean it – were we really better than last night?’ another asks. More than half an hour later – every fan request, picture and hug seen to – Sam Smith is on his bus. As the engine rumbles into life, I have time for one final question. This has all happened so quickly, I say – fame, fortune, awards, Madison Square Garden – all off the back of one album. Where do you see yourself in five years?

‘I’m going to open up a coffee and flower shop. I even know the name but I won’t tell anyone what it is,’ Smith says. He can see the scepticism in my eyes, and smiles. ‘I’m going to do it, 100 per cent. I don’t know when, but give it five years and I’ll do it. There’s a lot I need to go into. I’ve got to find the right coffee bean.’

Sam Smith's single Like I Can is released on December 8, from the album In the Lonely Hour