Mark Oaten has stood down as the Liberal Democrat's home affairs spokesman over an alleged affair with a rent boy.
He had been regarded as one of the Liberal Democrats' fastest rising Parliamentary stars and became one of the contenders to be the new party leader after Charles Kennedy's resignation.
Educated: Queen's Comprehensive school, Watford; Hatfield Polytechnic
Family: Married, two daughters
Before politics: Lobbyist
Key quote: "We need a leader with the courage of their convictions and the energy to revitalise liberalism for the 21st Century"
Mr Oaten, 41, was close to the former Lib Dem leader and had been planning to run Mr Kennedy's campaign for re-election.
Indeed he was at Mr Kennedy's side when he made the decision to stand down as leader and said he was "deeply disappointed" about the "whispering campaign" against his friend.
He contributed to the 2004 volume The Orange Book, which put forward a pro-market, less regulative agenda for the future direction of the party.
But Mr Oaten said nothing in his chapter about the use of markets in providing public services and instead addressed the issue of prison reform.
His pre-political life - while in business as a managing director of a PR firm - differed significantly from the city background of the other Orange Book contributors.
He has strongly opposed ID cards, and though he has shied away from older Lib Dem ideas about legalising cannabis, he has still said that the focus should be on targeting dealers and those users who resort to crime, rather than moderate drug-users.
As home affairs spokesman Mr Oaten became one of the most prominent of a new breed of Lib Dem MPs who favour "tough liberalism", mixing the party's concern for civil liberties with recognition of the rights of crime victims.
Last autumn he said the party would not give the government a "blank cheque" over its anti-terror legislations.
He told the Liberal Democrat annual conference in Blackpool that the plans to extend the detainment of terror suspects for up to 90 days and make "glorifying" terror an offence would damage civil liberties.
He said the Lib Dems would vote against these plans in Parliament - breaking the cross-party anti-terror consensus. Indeed Mr Blair subsequently lost his first ever Commons vote as PM on the 90 days issue.
Last month, Mr Oaten insisted that the Lib Dems would not change course in the face of a Conservative Party led by David Cameron.
As Mr Kennedy's leadership crisis deepened late last year, he sent an e-mail to party activists outlining his achievements in this role, which was seen as "pressing the green button" on his own bid for the top.
But within minutes of Mr Kennedy's first statement on his drinking and calling for his MPs to put up or shut up, Mr Oaten ruled himself out of running against his leader.
A skilled political operator and fluent Commons performer, the MP for Winchester has been rapidly promoted within the party.
In 2001, he was given a brief as chair of the parliamentary party and cabinet office spokesman. He was promoted to the job of Lib Dem shadow home secretary in 2003.
Previous roles in the Lib Dem team include spokesman for disabilities, a member of the foreign affairs and defence team, and chairman of the All Party Groups on Far Eastern Prisoners of War and Adoption.
He was one of the leading campaigners on compensation for former prisoners-of-war and is also interested in children's issues.
He successfully introduced a bill in the Commons tightening the laws on overseas adoptions.
Mark Oaten's election to the Commons in May 1997 marked a surprise gain for the party by the narrowest of margins - he was judged to have won by just two votes.
However, the defeated Conservative Gerry Malone refused to accept defeat and successfully petitioned to have the result overturned on grounds of irregularities.
The election was re-run in November 1997 and resulted in Oaten's majority increasing to over 20,000 votes. He held the seat in 2001 with a substantial majority. And last May, he held the seat with a majority of 7,476.
A former SDP councillor and a professional lobbyist, he supports the use of private finance in public services.
He lives in the Winchester constituency with his wife and two daughters.