Like a best man speech for a celebrity, the roast - where a famous guest of honour is mercilessly insulted by other celebs - is a long-standing ­tradition in the US. Channel 4 has imported the concept and tonight it's the turn of Sharon Osbourne - a human equivalent of an open goal.

Hosted by Jimmy Carr, this is the funniest and also the rudest hour of TV all week, with Jack Dee, Patrick Kielty, Gok Wan, Alan Carr, Louis Walsh, Ronni Ancona, Keith Lemon and Elton John paying acid-tongued tribute to Sharon's extensive plastic surgery, mothering skills and propensity for sending dog poo to her enemies.

And this put-down from Patrick Kielty shows that nothing is too near to the knuckle. "It's fair to say that Ozzy has never strayed," he quips. "He did once make a dash for freedom but after Sharon cut the brakes on the quad bike, he's now learned his lesson..."

Although it might not sound it, a roast is the ultimate honour for any entertainer. First started at the New York Friars' Club in the 1920s and made famous by Dean Martin on his TV show in the 1960s and 1970s, a roast involves you sitting in a room with your friends, family and colleagues as they mercilessly take the mickey out of you - goodnatured bullying, as the host Jimmy Carr calls it. There are three roasts this week - don't miss Sharon Osbourne's on Thursday and Chris Tarrant on Friday - but first up is Mr Showbusiness himself, Bruce Forsyth. Lining up to pay mocking tribute are Jack Dee, Jason Manford, Bruno Tonioli, Arlene Phillips, Jon Culshaw, Barry Cryer and, best of all, Jonathan Ross. It may be sycophantic, but there are still some hilarious moments.

For fans of excruciatingly painful television, don't miss Channel 4's Comedy Roasts in which Bruce Forsyth, Sharon Osbourne and Chris Tarrant are ritually insulted by visibly uncomfortable guests including Jonathan Ross, Sean Lock and Jack Dee. A longstanding American showbiz tradition, these British attempts are stilted and embarrassing. The Osbourne edition in particular is like a glimpse into Hell.

Although Graham Norton was in his finest form about five years ago on Channel 4, his squawky chat show moves tonight to BBC One. The first guests are that rare combination of the incomprehensible and the ageless, Ozzy and Sharon Osbourne, chat show perennial Ricky Gervais (who was on Jonathan Ross's show just a month ago) and ballad mistress Olivia Newton-John.