The BBC's John Sergeant reports: "The crucial endorsement came from William Hague"

The vacancy in one of the Tories' safest seats in the country is a result of the death of the maverick Tory MP Alan Clark at the weekend.

The announcement comes hours after Mr Portillo attempted to "clear the decks" before returning to public life by admitting he had homosexual experiences in the past.

The by-election date has not been set but it is expected before Christmas. If Mr Portillo is successful and returns to the Commons, it will spark much speculation about a leadership challenge against William Hague.

The Tory leader has publicly welcomed the Mr Portillo's move, saying he wanted the former minister back in the Commons.

He said Mr Portillo's admission of homosexual experiences was "essentially a private matter".

'A difficult decision'

In a statement, he said: "I have informed Conservative Central Office that I wish my name to go forward for the vacancy caused by Alan Clark's death.

William Hauge: "I will continue to encourage him"

"I haven't found this a difficult decision as to contest Kensington and Chelsea would be a great privilege for any Conservative. I have no doubt therefore that there will be many excellent applicants."

The effect of the revelations about his personal life on his attempt to represent Kensington and Chelsea are unclear, but the former MP for Enfield Southgate has urged the party to show tolerance.

He revealed his gay past in an interview with The Times, which took place in July but was only published on Thursday, after getting "fed up with all the innuendo".

Michael Portillo: "I would hope the public would want their politicians to be truthful"

Mr Portillo, who says he is now happily married, was asked detailed questions about "slurs" about his sexuality while at Cambridge University in the early 1970s.

"I will say what I want to say," he said. "I had some homosexual experiences as a young person."

'So what?'

Michael Portillo: "I'm not going to go back to public life and be dogged by false rumours"

Asked on Thursday morning whether he believed the Conservative Party was tolerant enough to accept his revelation, Mr Portillo said: "I very much hope so."

He added: "I think if I read this about somebody else, my reaction would be to say 'so what'. I hope that may be other people's reaction but we'll see."

Mr Portillo has insisted that "nothing of this sort" had happened while he had been in public life.

He said: "When I was in public life I was dogged by rumour, if I go back to public life I'm not going to have all that again.