Controversial Conservative MP Boris Johnson is to stand as a candidate for mayor of London.
The Henley MP said the opportunity was "too wonderful to miss".
Mr Johnson's decision came hours before the noon deadline for candidates for the Conservative nomination to declare their intention to stand.
He is now expected to be the most prominent runner in a small field of candidates hoping to seize the Tory nomination to take on Labour's sitting mayor Ken Livingstone in next year's election.
"The mystery is not why someone would want to be mayor of London. The mystery is why someone would not want to do the job and to try to make this city greater still," he told London's Evening Standard."A city as great as this deserves a great debate and a proper contest for its future."
Mr Johnson's decision, following weeks of agonising, comes as a boost to Tory leader David Cameron.
Mr Cameron had been struggling to find a high-profile candidate to take on Mr Livingstone, having failed to persuade figures including Greg Dyke and Sir John Major to throw their hats into the ring.
Although he has a reputation for gaffes and a complicated personal life, Mr Johnson is one of the Conservatives' most popular and best-known faces, thanks in part to his appearances on TV shows such as Have I Got News For You.
PA