His dramatic disclosure came on the eve of the presidential impeachment debate.

Washington Correspondent Paul Reynolds: "Surprising twist"

Mr Livingston said his past behaviour had almost cost him his marriage.

But in a reference to President Clinton, he said his indiscretions were not with his employees and he had never been asked to testify on oath about them.

His statement was first disclosed by a Capitol Hill newspaper, Roll Call, on its Website.

Shortly afterwards, Mr Livingston was given the support of his party colleagues, many of whom had no idea what was coming.

New Jersey congresswoman Marge Roukema said that after he read his statement, "there was a moment of silence when people looked across the room, and eyes met. It was nothing that anyone expected."

Bob Livingston and wife Bonnie at a gala two weeks ago

Another Republican representative, James Rogan said: "He accepted responsibility for his conduct and he got a standing ovation. I think he's a stronger leader because he showed character."

Mr Livingston has been responsible for drawing up the timetable for the impeachment debate in the House, which begins on Friday - despite Democratic objections that the issue should be left until military action in Iraq had ended.

Stephen Sackur: There is a poisonous partisan atmosphere in Washington

The House will debate Mr Clinton's behaviour in trying to hide his relationship with former White House intern Monica Lewinsky before deciding whether the president should face a trial in the Senate.

Mr Livingston said in a statement: "When I announced my candidacy for speaker, I said I was running for speaker, not sainthood. There was a reason for those words.

"I have decided to inform my colleagues and my constituents that during my 33-year marriage to my wife, Bonnie, I have on occasion strayed from my marriage. Doing so nearly cost me my marriage and my family."

Mr Livingston's spokesman Mark Corallo: "No talk of resignation"

Mr Livingston won the race to become speaker after Newt Gingrich in the aftermath of the Republicans' poor showing in the US mid-term elections.

His allies denied early reports that Mr Livingston was planning to resign - even before he has been sworn in as speaker - over the issue.

The Louisiana Republican added that he was aware that news organisations had been investigating his private life and that he would not be dissuaded from continuing with the impeachment debate.

Marathon session

Senate Majority Leader Trent Lott: "We will go to trial and there won't be any deal making"

Under the timetable drawn up by Mr Livingston proceedings will begin at 10am local time (1500 GMT) and could last up to 19 hours, with a vote likely on Saturday morning.

Members will discuss four articles against the president: two of perjury and one each of obstruction of justice and abuse of power.

With a Republican majority in the House and loyalties fiercely split down party lines, it seems likely that at least one article of impeachment will be approved.

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That would see Mr Clinton become only the second US president to be impeached and brought before a Senate trial.

And Senate Majority Leader Trent Lott has ruled out any deal to spare the president a trial in the Senate.