Relief for Tories as Boris Johnson opts to stay at City Hall

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Boris Johnson will stand for a second four-year term as Mayor of London, putting on hold any plans to resume his career at Westminster and succeed David Cameron.

Mr Johnson's decision to run again in the next mayoral election, in 2012, will come as a relief to senior Conservatives. However, some are worried that, in an attempt to keep his post, he will publicly oppose the Government over spending cuts affecting the capital and its proposed cap on immigration from outside the European Union. He is sympathetic to calls for an amnesty for illegal immigrants and described Mr Cameron's claims about a "broken society" as "piffle".

The Mayor is expected to say he would serve for a full term if he wins re-election in 2012. To fulfil any Tory leadership ambitions, he would have to be an MP, and that could mean leaving City Hall halfway through a second term.

Mr Johnson was once sacked from the Tory frontbench by Michael Howard for lying about an affair and some Tory MPs believe his colourful private life would prevent him becoming party leader.

Asked if he had ambitions to be Prime Minister, he said yesterday: "I'm more likely to be decapitated by a Frisbee or locked in a disused fridge. We have a very good Prime Minister. My job now is to negotiate with him and with the Chancellor the best possible deal for London."

Although Mr Johnson and Mr Cameron knew each other at Eton and Oxford – where they were fellow members of the hell-raising Bullingdon Club – they have been seen as rivals rather than friends in the political world. Mr Johnson, regarded as a potential future leader, was denied an expected promotion to the Shadow Cabinet after Mr Cameron won the Tory leadership in 2005. He languished in the post of higher education spokesman.

Mr Cameron, who admitted that Mr Johnson was better known to ordinary members of the public than he was as Leader of the Opposition, turned to him when the Tories needed a credible candidate to win London for his party. He ousted Ken Livingstone in 2008 after eight years as Mayor.

Declaring that he would be "crazy" not to seek a second term, Mr Johnson said: "With every day that passes, I have come to love and understand the complexities and challenges of the job, and I also see how much more there is to do. I will continue to work with every fibre of my being to accomplish the tasks we set ourselves in 2008."

Asked why he had delayed his announcement, Mr Johnson said he had wanted see how key initiatives worked, admitting to a "certain amount of nailbiting" over the scheme to make thousands of bicycles available for hire across London.

The former Henley MP played down his rift with the Government over its planned cap on immigration, saying he was merely lobbying for limits to be "thought through" so that businesses such as law firms and film companies were not prevented from bringing in the best talent.

Mr Livingstone is front-runner to be the Labour candidate in 2012, but must first defeat the former MP Oona King to win the party's nomination this month. He said: "I welcome Boris Johnson's confirmation that he will run again in 2012 because he now has a record that means he can be held to account. Boris Johnson cannot escape the fact that he has pioneered huge cuts in London and he vigorously campaigned for his Tory colleagues to win the general election, knowing full well the economic policy they would deliver and the damage they would do to policing and transport. The Government's cuts are his cuts."