When playing cameo roles in dire Hollywood clunkers in his twilight years, the actor John Gielgud perfected a visual expression that signalled: “It’s the money, dear boy. I’m not really in this film.” During the past week, the same look has often crossed the face of Simon Cowell, creator and judge of Britain’s Got Talent, the show that has taken television viewers by storm.

Cowell, not known for his sensitivity, clearly wondered what he had unleashed when he and fellow judges, the journalist Piers Morgan and actress Amanda Holden, were called upon to assess the merits of Rupert the piano-playing pig. Rupert’s star turn entailed going through a door, opening some miniature curtains and tickling the ivories.

The problem, as Cowell pointed out churlishly, was that “he’s not playing the piano, though, is he”. The pig was only stepping on the keyboard with his trotters. As events were to turn out, this porcine novelty was not the worst transgressor in a series that is a confection of tripe mixed with genuinely appetising morsels.

Anyone who thought variety was dead should tune in to Britain’s Got Talent and catch its live final tonight. Winnowed from thousands of entries, its nightly instalments have featured an erotic dancer who makes sparks fly off her pants, a boy who can make his ears squeak, a 6ft transvestite with a midget who becomes her horse, a 10-year-old stand-up comic and an octogenarian tap dancer.

Then there was the knife-throwing act of Brian and Melanie Webber, dressed in Red Indian attire as Little Firewater and White Dove, whose haphazard activities moved the judges to intervene. “We stopped you because we thought you were about to murder your daughter,” Cowell said.

Something about this dementia has infected the public, who have been watching in their millions at the expense of Big Brother and even Wednesday’s final of The Apprentice. Its nightly antics have increased ITV’s rating by 40%, and replayed clips have exceeded the most popular entries on the video-sharing website YouTube, some acts attracting nearly a million hits.

The series has tapped into a vein of public whimsy that runs back to Hughie Green’s talent show Opportunity Knocks, first shown in 1956 and revived from 1987-1990. Some even see Ant and Dec, the show’s larky presenters, as reincarnations of Morecambe and Wise.

Cowell, dubbed “Mr Nasty” and long associated with humiliation TV in his shows The X Factor, American Idol and Grease is the Word, has adopted a gentler format of end-of-the-pier entertainment. The new show adopts the formula of his hit America’s Got Talent, incorporating the refinements of reality television. Audiences vote by phone-in and the judges have buzzers that, if all pressed at once, instantly expel contenders into outer darkness. But this is a feelgood show, with an indulgent studio audience raucously supporting the most hapless acts.

Last week’s tumult culminated in three semi-finals, with eight acts performing to produce two finalists each night. After the public selected the best performance, the judges chose between the next two most popular acts. The victor in tonight’s final will win a £100,000 prize and a part in the Royal Variety Performance. Thanks to the culling rate, the Queen will be spared not only the piano-playing pig but such embarrassments as Pansy the singing poodle, whose yelp was matched by her mistress’s voice. “There are two barking creatures on the stage,” Morgan remarked unkindly.

Beyond the wackiness there have been enough arresting performances to make it a serious amateur talent contest. One of the most popular acts was Dominic Smith, a 14-year-old from Wednesbury, West Midlands. His version of Unchained Melody seemed to put him on the path to victory until Morgan, exercising his casting vote on Thursday, ejected him in favour of Damon Scott, a 27-year-old ventriloquist who had sung Michael Jackson songs with a puppet monkey called Bubbles. You couldn’t make it up.

Scott went into the final with Paul Potts, an operatic mobile phone salesman from Port Talbot with a heartbreakingly sad face who moved the audience to tears with his version of Nessun Dorma. He first sang opera for karaoke and went on to perform in a masterclass for Luciano Pavarotti, but days after recovering from a benign tumour he was knocked off his bicycle and broke a collarbone. Last week he became an overnight star, his performance seen by hundreds of thousands on YouTube.

After one entertainer was expelled for being a sex offender, Friday’s finalists turned out to be a breakdance group from Coventry called Kombat Breakers, and Bessie Cursons, an 11-year-old from Portsmouth who gave a winsomely energetic version of a Mary Poppins song. The judges, however, seemed more fascinated by a couple of “aerial entertainers”, Rebecca Peache and Donovan Jones, whose sinuous muscularity prompted Holden to ask if they had great sex. “Amanda, that was sex,” Morgan observed. For Cowell, it was all a bit slow: “Can you do it faster?” They were mesmerised, too, by Victoria Armstrong, a 29-year-old erotic dancer from Manchester whose act consisted of applying an angle grinder to her reinforced crotch and chest, producing a stream of sparks and a general rise in temperature. “My mate welded me a thong, then I popped down to B&Q to get myself a grinder and I was away,” she revealed.

Despite Morgan’s insistence that it was a contest for variety acts, the bookies were putting their money on singers. Potts’s main rival seemed to be a six-year-old singer with an equally poignant story.

Connie Talbot, from Sutton Coldfield, discovered she could sing while cheering up her terminally ill grandmother with a selection of songs from The Wizard of Oz. Her mother bought her a karaoke machine because the family could not afford singing lessons. After her rendition of Over the Rainbow on Britain’s Got Talent she was hailed as “the next Charlotte Church” and signed up to Cowell’s record label Sony BMG.

The novelty of Britain’s Got Talent is that it turns the audience and even the judges - barring the odd snide wisecrack - into nice people. Courtesy was revived by the veteran hoofer Bruce Forsyth as presenter of Strictly Come Dancing, in which seldom a cruel word is uttered.

But are more insidious elements at work? A recent Marxist analysis of reality television in International Socialism concluded a show such as Big Brother reproduced some of the “most pernicious effects of capitalism” in which “human energy and initiative are ruthlessly exploited in order to make money”. At the same time, the article maintained, such shows represent a dream of escaping from capitalism - “of transcending alienated labour, escaping from conformity and flowering as an individual”. Yeah, right.

The DNA of Britain’s Got Talent can be traced to the first American series of Candid Camera in 1948, featuring ordinary people instead of actors. Britain’s first reality show, in the modern sense, was the 1974 programme The Family, which followed the lives of the working-class Wilkins family of Reading.

The concept of putting strangers together and recording the ensuing drama was pioneered on Dutch television in a 1991 series called Nummer 28. Yet the idea of Big Brother struck its eventual British promoter, Peter Bazalgette, as repugnant. He rejected the concept in a letter to the programme’s Dutch inventor, arguing: “The rats-in-a-cage who’ll-do-anything for-money is something I doubt we could sell on to commercial television . . . as currently constituted, we feel the show has a narrow market in the UK.”

Some contend that shows like Britain’s Got Talent are the inevitable consequence of broadcasters’ search for a niche market after the collapse of mass audiences. These programmes can hold on to young people and they are cheap. A drama can cost about £875,000 an hour, whereas reality programmes can cost as little as £114,000 an hour. Ordinary people, and even minor celebrities, cost less.

Cowell’s stable of reality television shows has made him rich. His company, Syco was responsible for 40% of the profits of its parent, Sony BMG UK, in 2006, and this year’s Sunday Times Rich List values him at £100m.

The question is how sustainable Britain’s Got Talent and other reality shows will prove to be. Political correctness, unleashed by Jade Goody’s racist comments about Shilpa Shetty on Celebrity Big Brother, has contributed to Big Brother’s current fall from grace.

In Italy, the state broadcaster Rai has taken the unprecedented decision to scrap all reality shows next year. “After all the vulgarity, all the swearing and the smut, its time is finally coming to an end,” rejoiced the daily newspaper La Repubblica.

Could it ever happen here?