If you were to draw a straight line through the history of British comedy from Max Miller to Viz, it would pass through 29 films whose titles - with two exceptions - began with Carry On. Sizeist, sexist, racist, ageist, the Carry On films were one of the three most successful film series ever to emerge from a British studio, alongside the James Bond movies and the Hammer melodramas. English popular culture is vibrant with vulgarity. But the Carry On's were the aristocracy of smut.

Reviled by the buttoned-up middle classes and by liberal intellectuals alike, the series finally fizzled out in 1978 with Carry On Emmanuelle, a last attempt to breathe life into a corpse that had been cooling since 1969. But now the Carry On opus may signal the salvation of the British film industry. Next month, the 30th Carry On film goes into production at Pinewood Studios after a hiatus of 14 years. Carry On Columbus will be directed by 72 year-old Gerald Thomas - brother of Ralph (director of Doctor in the House), uncle of Jeremy (producer of Sheltering Sky) - who has directed every Carry On movie ever made. The £2 million film is to be executive-produced by Peter Rogers, another Carry On veteran, whose statement "We're vulgar but we're never crude" was the bewildering credo of the genre.

It was appropriate that the great days of the Carry On films coincided with the Golden Age of James Bond. Innuendo stood in for sex before Swinging London really took hold in the late-Sixties and early-Seventies cinema. Thus Sean Connery's smart-ass one-liners as 007 played on sexual punning to the hilt in much the same way as Talbot Rothwell's innuendo-riddled Carry On scripts. Rothwell, who, coincidentally, lived in a village called Fulking, understood the fine line: people should endlessly go on about sex, but never succeed in having it. The more prim and proper a character, the more vile their innocent double entendres (of Joan Sims: "My! That is a big one you've got there Mr Boozey"). In Don't Lose Your Head, the aristocratic Joan Sims drinks champagne with Kenneth Williams. "Sorry, the bubbles lodged in my chest," she burps. "Yes there's room for a couple of lodgers in there", snorts Williams. No other writer has come close to Rothwell's delicate way with the indelicate.

In purely structural terms, the films were perfectly judged. Too crude and the audience would recoil. Too tame and the audience would snore. More importantly, the Carry On films were, literally, carrying on a tradition of British humour that first emerged on the stages of the music halls. Significantly, all the leading actors had music hall backgrounds.

The films would be much the poorer without their extraordinary gathering of grotesques. Sadly, there are few of the old stalwarts left. Kenneth Williams, whose greatest moment came in his portrayal of the French revolutionary, Camembert (The Big Cheese), in Don't Lose Your Head. Sid James, who gave us an immortal Mark Antony, likened by film lecturer Robert Murphy to a gone-to-seed Richard Burton. Hattie Jacques, the perpetual hospital matron, whose every step was accompanied by a trombone theme. Charles Hawtrey, the bespectacled weed, usually coerced into cross-dressing; Peter Butterworth, the epitome of overweight cunning and suburban lechery…all of them immortalised on film and in the memory of true movie-goers of the Sixties. Hattie Jacques said that walking onto a Carry On set was like going back to school after the holidays. "All your friends were there."

None of the surviving stars - Barbara Windsor, Bernard Bresslaw, Joan Sims - talk about the series any more. They had to fight hard in their careers to get away from it. This week, Barbara Windsor, of the corncrake voice and wiggle cooed: "Ooooh, I must have talked about this 25 million times. I only did seven and I get a reputation as a Carry On girl." She has been approached about the new venture, but is being discreet. "I really can't talk about it, darling. They're in the middle of casting it. They've just got the scripts in the post, though it's very late. It starts in three weeks." It was ever thus.

The four-week shooting schedule is tight even for a Carry On. Gerald Thomas previously completed shoots in a break-neck six weeks. Good lines were not given preferential treatment. "None of them was a pearl of wisdom anyway", he explained. Not pearls maybe, but often priceless: consider Williams's Julius Caesar. As he is stabbed, he cries: "Infamy! Infamy! They've all got it in for me!" The business of humour in a Carry On film was to pile on as many jokes - good, bad and truly appalling - as possible. They worked largely on the bulldozer principle. One or two of these gags in isolation are not funny. Hundreds of double entendres coming at you over 90 minutes and surrender is the only option.

The early plots were deceptively tight, low-rent adaptations of classic farces. Civic and corporate corruption, innocence betrayed and ultimately triumphant, social hypocrisy unmasked, were the frequent thematic concerns. No wonder the middle classes threw up their hands in horror - it was their story. The Carry On's dealt with the horrors of package tours (Carry On Abroad), the first minicab firm run by women (Carry On Cabby), the British occupation of India (Carry On Up The Khyber) all accompanied by a barrage of farts, belches, squeals and Sid James's laugh, the dirtiest on film. Pompousness was pricked, vulgarity was paraded as a virtue and sex was the Grail for these bargain-basement Knights Templar.

But the Carry On's lost their innocence in 1969 and sowed the seeds of their own, albeit temporary, downfall. As Andy Medhurst indicates in his book, Music Hall and British Cinema: "By their increasing sexual directness…they put an end to the very traditions of innuendo that sustained them for so long. After Barbara Windsor's brassiere burst (in Carry On Camping, 1969) where was the humour in teasing about the possibility of such an occurrence?" It wasn't so much that the films changed, though they became bluer towards the end. It was that, like recalcitrant children, they refused to grow up.

Today, like Benny Hill, they seem curiously innocent, their blatant stereotypes too exaggerated to be offensive. To some, they're a welcome relief from the heavy hand of Political Correctness. And with Aids dictating a new restraint in sexual behaviour, innuendo and tease are back in style. In terms of film history, the Carry On's are a little genre all of their own. Consequently, they have gradually crept into the art houses. Both the Electric and the Scala have staged Carry On festivals. They pop up at the NFT. Their appeal has begun to spread to a generation who weren't even born when the series started in 1958.

Carl St John, founder of the Kenneth Williams and Sid James Society, welcomes the new interest but fears Carry On Columbus may not live up to the earlier standards. "From our side, the Carry On team were the films", he sighs. "They've gone now. They were the gang. Doing a film without them would be like doing Clouseau without Sellers." But, he adds: "I think the films deserve the critical attention they perhaps didn't have at the time. Good scripts, good actors - and they'll carry on making new generations laugh."