Boris Johnson’s proposals for an amnesty for longstanding illegal immigrants have been criticised by David Cameron, raising questions about who has the final say over his policy agenda.

Mr Johnson will say this week that British citizenship should be offered as part of a one-off amnesty to immigrants who have been in Britain for more than four years.

The move will come as a surprise to many on his own side, as well as many Labour MPs, who have both regarded the topic as politically toxic and attacked the Liberal Democrats in the past for suggesting it.

Mr Johnson will suggest that a reprieve should be granted to those who overstay their visa, or asylum-seekers whose claims have been rejected but who are unable to return to countries such as Zimbabwe.

But Mr Cameron made it clear yesterday that he was unhappy with the proposal, at the same time stressing that he would not overrule his party’s candidate for London.

Mr Cameron said: “No, I don’t agree with that. The problem with amnesties is that they just store up another for the future, as people expect another one. Boris is his own man. He is standing on his own platform and he dictates his own policies.”

Mr Cameron’s principal link with the Johnson campaign is understood to be Nicholas Boles, a former think-tank director and one-time contender to be Tory London Mayor.

He is helping to form a “transition team” for City Hall should Mr Johnson win the race and is smoothing relations between Mr Johnson’s campaign team in County Hall and Conservative headquarters. Mr Boles’s principal task is to find a chief of staff for Mr Johnson, though The Times has been told that he may step into the role himself if no suitable candidate can be found before May 2, when the count is completed.

Mr Johnson claimed yesterday that he was the victim of an “absolutely ruthless” dirty tricks campaign aimed at derailing his bid to become Mayor of London.

In an interview with The Sunday Telegraph, the Tory candidate accused his opponents of deliberately “lying” to misrepresent his policies.

Campaign aides also said that internet hackers had broken into his e-mail system, bringing down his computer system for several hours last week.

The paper said that there was no suggestion that Ken Livingstone, his Labour opponent, was in any way involved in the alleged dirty tricks campaign.

This comes as an anonymous viral internet video was released attacking Mr Johnson and sent by e-mail to journalists covering the election.

Ironically entitled “I Love Boris”, the two-minute music video called the Tory mayoral candidate a “buffoon” and “incompetent” and shows Governor Arnold Schwarzenegger appearing to criticise Mr Johnson at a conference.

The Livingstone campaign denies any connection to the video, made by a group which calls itself TRSG. The high production standards have raised suspicions that the video had been financed professionally.

Tessa Jowell, who is running Mr Livingstone’s campaign, defended her policy of banning senior government figures from referring to the Tory mayoral candidate as simply “Boris”.

Ms Jowell has instructed ministers to call him by his full name, Boris Johnson, or just to refer to him as “the Conservative candidate”.

Those caught breaking the order were told that they would have to pay £5 each time into a “swear box” — with Jack Straw, the Justice Secretary, said to have been the first to fall foul of her edict.

“What we have to avoid is a situation where people think this election is a joke and that the future of London is not serious,” Ms Jowell told Sky News’s Sunday Live programme. “Our argument, made publicly at every possible turn, is that Boris Johnson’s policies for London are not serious.”

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