Boris says he'll challenge 'King Newt' for London Mayor

His decision to stand as a Conservative candidate for the job brought palpable relief to Tory HQ, ending the party's long search for a strong personality to enter the contest.

One of the party's most popular and well-known MPs, he will remain the member for Henley during the campaign but resigned from his frontbench job as Tory higher education spokesman.

A senior party source said David Cameron was "prepared for gaffes" from the accident-prone MP, but added: "Boris is one of those rarest of politicians who seems to be able to get away with anything."

Mr Johnson will this week hit the campaign trail in Ealing Southall, ahead of Thursday's crunch by-election in the constituency in which the Tories are under pressure to do well.

He will then launch a "viral" campaign website featuring videos of himself talking in characteristic fashion about his efforts to topple Labour's Mr Livingstone.

He launched a pre-emptive strike against the "great King Newt", who he said was squandering the Mayor's £300 share of Londoners' council tax bills.

He vowed to tackle problems in housing, transport and crime in order to "put the smile back on people's faces".

Surrounded by a media scrum as he pushed his bike outside City Hall in the capital, he said he was "thrilled and excited".

Mr Cameron must remain studiously neutral - in public at least - since his party is holding an open primary process to select a candidate.

But senior Tories have been desperate to find a high-profile challenger and believe Mr Johnson's broad appeal means he will have a real chance of ousting Mr Livingstone in the 2008 race.

Current London Mayor Ken Livingstone

Around 50 candidates have put their names forward, and a shortlist of around ten will be invited to go for full-scale interviews before being whittled down to between two and five names.

A series of hustings will be held throughout the summer before all Londoners - whether Tory supporters or not - are invited to vote on which candidate should go forward.

The result will be announced at the end of September on the eve of the Tory's annual party conference.

Mr Johnson's frontbench career appeared to have stalled after a series of gaffes and Mr Cameron's apparent sensitivity to claims that he has too many fellow Old Etonians in his shadow Cabinet.

His announcement, after weeks of agonising, came only a few hours before Monday's noon deadline for applications.

He said: I want to bring fresh ideas to the capital and offer a new direction for Londoners.

"Even the greatest cities have further greatness in them. I will stand for a greater London and for putting the smile back on London's face."

Mr Livingstone said: "To put someone in charge of London with such a right-wing record, who has no experience of managing anything practical at all, and who has shown no serious interest in even the most important issues confronting the capital, would not be a joke, but seriously damaging for London."