Boris Johnson is to stand down as MP for Henley "as soon as possible", his team said at the weekend.
Following his election as Mayor of London, Mr Johnson will talk soon with Conservative officials to arrange for his departure from the House of Commons.
His resignation will spark a by-election in one of the Conservatives' safest seats, which he has held since 2001.
A spokesman for Mr Johnson said: "As soon as he can stand down, he will do so.
"He will have to talk with the party, but in the next couple of days it will all become clear."
Mr Johnson succeeded former Deputy Prime Minister Michael Heseltine as MP for Henley at the 2001 general election with a majority of 8,458.
He was re-elected in 2005 with an improved majority of 12,793 and a 53 per cent share of the vote.
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