All together now... lights down, bubbles on!

A Slice of Britain: Hot Tub Cinema sees scores of young film fans flocking to watch movies in (very) close company with scantily clad strangers, on a rooftop near you

I can rave in a laundrette, drink a pint in a multi-storey car park, and watch music videos in disused petrol stations, if I so wish, but not until now has the capital's penchant for pop-ups driven me to share a hot tub with total strangers.

For the past month, this is exactly where scores of London's less self-conscious, mostly 20- to 30-year-olds, all in search of something "unique" and "imaginative", have been flocking. Hot Tub Cinema, currently perched on the roof of east London's Netil House, boasts 12 hot tubs, a projector and a screen.

The idea is simple. You fork out a minimum of £22, strip off to your undies, or something slightly more waterproof, share a hot bath with a maximum of seven other scantily clad people, and then take in a film, in 40-degree heat, from a rooftop, with only bubbles to distract you from your neighbour's leg as it grazes yours. Scepticism is understandably high; I was asked if I had had a precautionary tetanus shot when I told friends I was to take the plunge.

But its popularity is harder to ridicule. Sold out at every event, numbers have doubled since the first showing, with 85 people taking to the tubs this weekend to watch Back to the Future. The owner has had to increase capacity by 50 per cent, and Future Shorts, the company behind Secret Cinema, is hosting its short-film festival in the tubs on Tuesday.

For Asher Charman, the 27-year-old founder of Hot Tub Cinema, it's a no-brainer. "They've been going like hot cakes since day one," he says, as he proudly shows me the wooden tub he and his dad built. "It's a genuinely unique combination of experiences. The film gives it all focus – without it, you just have a hot tub on a roof."

That aside, there is no mistaking the main attraction. As the titles begin to roll and people strip off in trepidation, squeals come from far and wide. In typically British manner, even the hot tubs are divided by price. There are three "cosy" tubs – nicknamed "the ghetto" by punters – where up to eight cram in side by side. There are four "spacious", designed for no more than six apiece, and the rest are for group hire of up to eight people, at £200 a pop.

Sharon McGoldrick, 33, a graphic designer from south London, is sharing a "cosy" with a friend. All laughs and giggles, she seems to be on the lookout. "A hot guy in my office mentioned something about hot tubs in London. I Googled it and this is what I found," she tells me. "I still don't know if he's coming."

Curled up in my own "spacious", I meet my five new tub mates. Three friends, in their late twenties, from all across London, and a model and an engineer, on their first date. As the wine flows, so do the jokes. "You're getting your clothes off early, aren't you, for a first date?" teases Paul Metcalfe, 27, a lawyer living in Moorgate. "I thought there'd be lots of people around, if the date was rubbish," laughs 25-year-old Andy Hadland.

But it appears there was no fear of that. While I am left trying to close my "dry box", thus ensuring the waterproofing of my possessions, the bubbles start, and any inhibitions are promptly forgotten. My dating couple start to kiss, wine glasses are swapped for shared bottles, and London's skyline looks increasingly majestic.

If there was ever any sense of tub hierarchy, by the time Michael J Fox sings "Go Johnny Go", it is nowhere to be seen. As bikini-clad women dance in the middle of their tubs, there are impromptu shouts of "hot tub relay", where punters jump from tub to tub, and water fights follow.

The dippers themselves are hard to categorise. While Netil House is a beacon of east-London warehouse-style cool, Hot Tub Cinema seems to attract a slightly more affluent mix of advertisers, financiers, lawyers and creatives. Some are from Essex and Reading, others have crossed the Thames in a bid to experience a perfect hot-tub movie moment, while the call of warm water and rooftop space has even attracted a number of Aussies, perhaps pining for home.

A local copywriter, Danni Emery, 30, from Clapton, east London, is wearing a swimming cap. Almost missing out on the event, because she thought it was a "little pricey", she describes it as "absolutely wicked". Her only complaint: the film wasn't water-related. She recommends Jaws, Splash, or Hot Tub Time Machine.

As for Charman, he seems thrilled. Without a hint of irony, he turns to me and says: "That's all I wanted really – for everyone to feel like we're all in this together."

Top hot-tub movie moments

1. Tequila Sunrise

It's 80s glamour all the way as Michelle Pfeiffer and Mel Gibson mark one of the most memorable movie hot tub love scenes to date.

2. Ferris Bueller's Day Off

Ferris takes to eating Oreo cookies in the tub with his girlfriend Sloane.

3. Austin Powers: International Man of Mystery

Austin is soaking with spy Alotta Fagina, when he breaks wind.

4. Charlie Wilson's War

Texas Congressman Charlie Wilson, (Tom Hanks), entertains women in a glitzy black and gold hot tub.

5. Lion King III

The third Lion King film proves hot tubs are not just for humans as Timon and Pumba, the animated meerkat and warthog, take a dip.

Additional reporting by Rachel Roberts