David Cameron slaps down Boris Johnson over call for amnesty for illegal immigrants

Mr Johnson said the capital would benefit from offering migrants a one-off pardon, forcing the Tory leader to distance himself from his candidate for London mayor.

Labour critics seized on the row as evidence that Mr Johnson's tightly controlled campaign could turn into an embarrassment for the Tories before polling day on Thursday, May 1.

Mr Johnson has built up a solid poll lead over Ken Livingstone and his aides are desperate to ensure that his notorious capacity for blunders does not undermine his campaign.

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Mr Cameron is pinning his hopes on a victory in the London race to deliver what he hopes would be a killer blow to Gordon Brown's ailing premiership.

But yesterday he slapped down the latest policy suggestion from his fellow Old Etonian, warning it would lead to demands for further amnesties.

Mr Johnson's aides confirmed that he backs giving failed asylum seekers and migrants who have been in London for at least four years the right to become citizens.

The mayor has no power to decide immigration policy, but can put governments under pressure on sensitive issues.

Mr Johnson's aides said he believed it would be good for London to regularise the position of thousands of foreigners who are working on the black market.

Giving them citizenship would allow them to work legally, pay taxes, and submit to integration classes to improve their English.

They said Mr Johnson had recently attended a citizenship ceremony and he believed that an amnesty would allow greater transparency on the true state of immigration in London.

But Mr Cameron said the idea of an amnesty "is not right" and ruled out adopting it as party policy.

The Tory leader tried to play down potential embarrassment by stressing that Mr Johnson was entitled to his own views on policy and was not bound by central party policy.

"The problem with an amnesty is that it just stores up another problem for the future as people expect another one," Mr Cameron said.

Mr Johnson claimed yesterday he was the victim of an "absolutely ruthless" dirty tricks campaign.

In an interview with the Sunday Telegraph, he accused his opponents of "lying" to misrepresent his policies.

The latest example appeared to be a spoof 'I love Boris' musical video circulating on the internet initially attributed to Mr Johnson's supporters.

But it emerged that it was put together by Mr Livingstone's backers in an attempt to draw attention to Mr Johnson's upper-class background.

His campaign claimed internet hackers had broken into his email and brought down his computer system for several hours last week.

"They are literally going round houses knocking on the door and lying," he said.

"There's been lots of sub-radar stuff, really abusive. I was quite surprised by the complete intellectual dishonesty in some of the ways they've tried to misrepresent me."

The Cabinet minister responsible for Labour's London campaign confirmed that ministers have been banned from referring to Mr Johnson as 'Boris' - because they believe the habit is adding to his popularity with voters.

Tessa Jowell has ordered ministers to call him by his full name or to refer to him as 'the Tory candidate'.

Those who do not must put £5 into a 'swear box'.