London Mayor Boris Johnson dismissed the £250,000-a-year he earns from a second job as "chicken feed".

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Mr Johnson also insisted it was "wholly reasonable" for him to write newspaper columns on the side because he did them "very fast".

The comments risk infuriating millions of Londoners struggling to make ends meet amid the economic downturn.

And they are unlikely to please David Cameron, who has ordered his shadow cabinet to give up extra work in the run-up to the general election to show their "commitment".

Mr Johnson, who is paid nearly £140,000 for his day job, was quizzed over his lucrative contract with the Daily Telegraph during an interview for the BBC's HARDTalk programme.

He responded: "It's chicken feed."

Pressed on whether voters would agree with that description, the mayor said he was being "frivolous".

But he went on: "I happen to write extremely fast. I don't see why on a Sunday morning I shouldn't knock off an article, if someone wants to pay me for that article then that's their lookout and of course I make a substantial donation to charity.

"Maybe that money shouldn't go to charity, maybe you'd rather I didn't make those contributions to charity. It seems to me to be a wholly reasonable thing to do."

Mr Johnson said: "I think that frankly there's absolutely no reason at all why I should not, on a Sunday morning before I do whatever else I need to do on a Sunday morning, should not knock off an article as a way of relaxation."

Mr Johnson decided to continue with his columns for the Telegraph after being elected last year, but donates £50,000 from his annual fee to charities.

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Asked whether Mr Cameron approved of his extra work, the mayor seemed oblivious to the instructions issued by his party leader.

"Well I'm not a member of his shadow cabinet and I'd like to see what the detail of that is," he said.

Labour's deputy leader on the London Assembly, John Biggs, said: "Londoners struggling through the recession will be astounded that their Mayor is so out of touch with reality.

"A quarter of a million pounds may be chicken-feed to him but the overwhelming majority of people in the capital can only dream of earnings like that.

"How can we have any faith in a Mayor who believes earning more than 10 times the majority of those whose interests he is supposed to represent amounts to nothing more than chicken-feed?"