Corny comics laughing all the way to the bank

"What's there to sell out? I'm a comedian," funnyman Rob Brydon recently said after being asked about whether he's lost any credibility after doing those diabolically unfunny breakfast cereal adverts – with the grim tagline "There's tasty, and there's Crunchy Nut". Brydon, à la Peter Sellers in Dr Strangelove, takes on three roles: smug TV presenter, harassed producer and Dick Emery-style trolley lady. He fails to steal a laugh on all counts. It's dismal and he no doubt pocketed an unapologetic sum for it.

Of course, there's little point bemoaning the amount paid to comedians for doing these (invariably laugh-free) commercials, but surely Brydon is wide of the mark to suggest that as a comedian he basically has no credibility to lose? And, more crucially, those commercials have – in my eyes, at least – made Brydon markedly less funny. And he's not the only one. Of late, an indecent legion of comedians/comic actors have embraced the filthy lucre of commercials, sullying their reputations and their "funniness". In no particular order the worst culprits are Chris Addison, Alexander Armstrong, David Mitchell, Stephen Mangan, Paul Whitehouse, and John Cleese (a serial offender for many years).

Ever since doing those Direct Line home insurance ads – "Shark Attack" with Amelia Bulmore and "Air Golf" with Alexander Armstrong (a comic actor who has already shot his comic bolt with those tiresome "Pimm's O'Clock" commercials) – it is frankly impossible to find Chris Addison quite as funny again. This is a stand-up comic who, in his smart routines, often rails against hypocrisy and stars in the biting satire In the Loop. What – apart from the obvious allure of pots of cash – possibly persuaded him to audition for and then star in an insurance ad, where he gets to play a personality-free, gormless salesman? Of course, it's the money, stupid. It's reminiscent of Woody Allen's gag about being asked to do a vodka ad. He's offered $50,000 but his conscience is plaguing him, so he asks his rabbi's advice and he tells him, "Don't do it". So he passes the ad up. A month later Allen opens Life magazine and spots his rabbi on a Jamaican beach with a "cool vodka in his hand".

At least Allen's rabbi found glamour, and at least those silly Leonard Rossiter and Joan Collins Cinzano ads of the 1970s had a hint of exotica and sauciness about them. Much more so than the Driving Standards Agency's Highway Code commercial, which David Mitchell (star of the exquisite Peep Show and numerous panel shows) lends his voice to. But worse still was Mitchell's decision to provide the voiceover for a £1m government commercial for "FRANK" that warned of the perils of cocaine. Mitchell voiced Pablo, the drug-mule dog. How can Mitchell ever have the gall to satirise anything ever again after lending his voice to those hectoring ads?

John Cleese has flogged his Basil Fawlty persona to death promoting unfunny ads for Compaq computers, Sainsbury's, Intel, Lexus, Schweppes, Titleist, Intel Centrino, Accurist, Magnavox TVs, and, worst of all, the recent AA offering.

Possibly the most grim example of a comedian doing a commercial is the gifted impersonator Paul Whitehouse, who lends his melancholy, silent-comedy era face to the Aviva campaign. To be fair, he's funny – in the "Green Army" ad, in particular – but it is reminiscent of Bill Murray's Bob, a despondent actor promoting Suntory whisky in Lost in Translation. A spent comic force reduced to advertising booze for a living. "Getting paid two million dollars to endorse a whiskey when I could be doing a play somewhere," Bob laments. How much did Brydon, Addison et al get to sell pieces of their funny bone to the advertisers?