Channel 4 has been a springboard to movies for some of the biggest names of British comedy, but nothing has leapt so high, so fast and so unexpectedly as “The Inbetweeners.”

With a staggering £30.3 million ($49.7 million) from 15 days, Film4’s “The Inbetweeners Movie” has broken records for the biggest U.K. opening weekend by a live action comedy, a TV spinoff and a British indie.

It posted the second highest opening of 2011, behind “Harry Potter and the Deathly Hallows: Part 2.”

That’s a remarkable result for a cult TV show that began just three years ago on Channel 4’s niche digital service E4 with a mere 321,000 viewers for its first episode.

But the show rapidly emerged as the defining sitcom of its generation for teenagers. This foul-mouthed yet warmhearted gross-out comedy, about the tribulations and humiliations of four sex-obsessed suburban schoolboys, has become a cultural phenomenon, introducing new obscenities (“clunge,” “bumder,” “bus wanker”) to the language

The third and final season, which aired last fall, drew an average audience of 4.1 million, with another 9.2 million views to date via Channel 4’s online service and YouTube. The three series, totalling 18 episodes, have sold more than 1.8 million DVDs in various box-set combinations.

Such figures explain why Channel 4 was so quick to fully finance the $7.5 million movie. It’s the pubcaster’s largest single investment in a movie since it downsized its Film4 unit nine years ago.

“The Inbetweeners Movie” follows the four protags on a debauched Greek holiday chasing girls after their graduation from high school. The budget came partly from Film4, but principally from 4Rights, the web’s commercial arm, whose DVD label, 4DVD, has made millions from the TV series.

While other distribs were skeptical about taking theatrical alone for a TV spinoff, Nigel Green at Entertainment Film Distributors came aboard at script stage to handle the U.K. cinema release, putting up the substantial P&A but no advance.

Although the Film4 logo is prominent at the front of the movie, the unit had relatively little creative involvement in a project far different from the kind of upscale auteur fare it usually backs. Its execs were happy to step back and allow writers Iain Morris and Damon Beesley, director Ben Palmer and producer Chris Young to continue their existing relationships with C4’s head of comedy Shane Allen and with Mike Morris, managing director of 4Rights.

Young was determined to avoid the usual lengthy film development process, which might have delayed the project and changed the essence of the show to something more high-minded but less funny.

“We had a very clear idea that we wanted to make a U.K. teen comedy along the lines of ‘American Pie’ or ‘Superbad,’ and we didn’t want anybody telling us how to do that,” he explains. “We were very arrogant about that, and we had an existing strong relationship with 4DVD, so we resisted the idea of getting into a whole new set of relationships.”

Speed was essential to capitalize on the success of the TV show and to hit an August 2011 release date, with the DVD following at Christmas. The team started shooting footage in Crete last August, before there was even a completed script or production coin in place. 4DVD bankrolled the trip in return for an option to fund the movie, which started principal photography in January. “Our relationship with 4DVD meant they could take a punt on that in a way that Film4 was never going to do,” Young says.

Nonetheless, Young praises Film4 head of production Tracey Josephs for providing expertise in production finance, without putting a brake on the autonomy of the writers. The deal also allowed the Young, Morris and Beesley to share ownership with C4.

For Film4 topper Tessa Ross, “The Inbetweeners Movie” is a triumph for her longstanding policy to connect the film unit more closely with the rest of the channel, by reaching out to other departments and giving them the support to take ideas and talent from the small- to the bigscreen.

After suffering a 20% budget cut shortly after its global success with “Slumdog Millionaire” in 2009, Film4’s fortunes rebounded last year with the arrival of C4 chief exec David Abrahams. He restored the cut and then added another 50% on top of that, bringing the budget up to $25 million a year. “The Inbetweeners Movie” is a repayment of his faith in the unit, and will strengthen Film4’s standing within the channel.

Foreign presales from a Cannes promo were surprisingly robust, given that the movie is essentially the final episode of a TV series with a modest international profile. Young also plans to explore interest from territories such as France and Australia in shopping remake rights. A theatrical deal is reportedly close in the U.S., where BBC American airs a sanitized version of the show, and MTV is prepping a 12-part remake.

Everything ‘Inbetweeners’

Writers Damon Beesley and Iain Morris produced the earliest TV work of Ricky Gervais, who named the two pretentious BBC producers in “Extras” after them.

“The Inbetweeners” was the first TV project for veteran Isle of Skye-based Scottish film producer Chris Young, whose last movie, “Seachd: The Inaccessible Pinnacle,” was in Gaelic.

E4 ratings: season one (2008), average 0.5 million. Season two (2009), 1.5 million. Season three (2010), 4.1 million (highest ever for E4).

Online, season three has 7.2 million views on Channel4.com, and more than 2 million views on YouTube.

The first weekend for “The Inbetweeners Movie” ($21.7 million), beat several records — for a comedy (previous best, “Bridget Jones: The Edge of Reason,” $17.2 million); a British indie (“Chicken Run,” $6.33 million); and a TV spinoff (“Sex and the City,” $14.4 million).

“The Inbetweeners Movie” has already overtaken the final U.K. totals for “Ali G Indahouse” ($16.9 million) and “Kevin and Perry Go Large” ($17.3 million) and “Borat” ($39.8 million). The next targets include top Film4 pic “Slumdog Millionaire” ($52.1 million), top U.K. comedy “Bridget Jones: The Edge of Reason” ($59.2 million) and all-time U.K. indie champ “The King’s Speech” ($75 million).