Only people with an appetite for rank theatrical rubbish will want to gorge themselves on this relentlessly boring little farce by Toby Young and Lloyd Evans.

Dismiss all memories of that artful huffing and puffing in advance of the first night by publicists, who hinted the authors had put living royalty up for controversial mockery by making them fun figures in a sex romp.

Actually A Right Royal Farce has all the shock-value of a pair of pubertal schoolboys caught fiddling with each other in the bike shed.

Hard upon the Queen's demise, Richard Keith's neither thick nor thuggish Prince Harry schemes like Shakespeare's Richard III, though without the laughs, to steal the throne from both his father and brother. After no end of plotting, of an unamusing and contrived sort, he comes unstuck.

Alex Bartram's amazingly lookalike, sympathetic William is restored to his rightful place as the farce, which ought to reach heights of climactic lunacy like the classic See How They Run at the Duchess, fizzles out. Few shows of such embarrassing, authorial ineptitude can have hit the London stage since the Blitz.

When it comes to embarrassment or bad taste, though, Young and Evans, who cocked several successful snooks at David Blunkett's sex life in last year's far better Who's the Daddy, score a bull's-eye.

The authors depict William as a doped, unstable nut-house candidate, who is dragged off to the funny farm after Harry has his own Australian girlfriend masquerade as Diana, speaking to her older son from Heaven. This plotting device strikes me as a mirthless, unpleasant exploitation of the Prince's grief for his dead mother.

Christopher Woods's cheap set, with its monogrammed carpet and gilt walls, with champagne bottles stacked on the floor, looks a suitably caricatured version of the real Palace.

Young and Evans, whose biggest success as farce writers is their obliteration of the intelligence, command and humour that they display as Spectator theatre critics, never manage to create any farcical momentum or classic spiral of panic or even sharp dialogue. Farce at its best is a wild offshoot from real life, not here.

Harry plots for the crown. One of his efforts involves dressing up as the Archbishop of Canterbury to deceive Andrew C Wadsworth's daft, simple, plant-hugging Charles into renouncing the throne. This characteristic scene could not be less funny.

Alan Cohen's slack production, with sudden black-outs, offers dull, far from pointed caricatures of members of the Firm. William Hoyland's prowling, priapic Duke of Edinburgh roves stylishly about and beds Sara Crowe's ridiculously vamping affected Camilla, then tries his luck with Harry's mysteriously Australian girlfriend Anoushka (Katie Beard).

Philip's exploits distract attention from the main succession business. The Duke may have one gorgeous, suitably acerbic line, when he mentions one of the African leaders of whom he has inquired 'Have you eaten any members of the opposition recently?'

Otherwise, though, A Right Royal Farce has all the bite and provocation of a toothless old corgi.