What's to sell out? I'm a comedian," funnyman Rob Brydon recently maintained after being asked about whether he's lost any credibility after doing those diabolically unfunny Crunchy Nut cornflake adverts – with the grim tagline "There's tasty, and there's Crunchy Nut". A gamey 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 it no doubt pocketed an unapologetic Brydon a tidy sum.

Of course, there's little point bemoaning the amount given 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, much 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 of many years standing).

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 rallies 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 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 who 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 in the 1970s had a hint of exotica and sauciness about them. Much more so than the Driving Standards Agency's "The 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, which 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 these hectoring ads?

John Cleese has flogged his Basil Fawlty persona to [a dead parrot] death promoting a plethora of unfunny ads: Compaq computers, Sainsbury's, Intel, Lexus, Schweppes, Titleist, Intel Centrino, Accurist, Magnavox TVs, and, worst of all, the recent AA ads.