The Liberal Democrats today begin the search for their third leader in less than two years after Sir Menzies Campbell, at the helm of his party for just 19 months, dramatically resigned.

The Liberal Democrats today begin the search for their third leader in less than two years after Sir Menzies Campbell, at the helm of his party for just 19 months, dramatically resigned.

Senior party sources explained that it was the corrosive speculation about his leadership and the prospect of it continuing for at least 18 months until the next General Election that finally made the 66-year-old MP for North-East Fife call it a day and tender his resignation.

His unexpected decision also comes in light of a resurgent Conservative Party under David Cameron's leadership, which has stolen some LibDem clothes, notably on green taxes and civil liberties, and a consequent slump in the polls for his own party.

Sir Menzies consulted colleagues over the weekend about his future and at a hastily-arranged meeting at the party's Cowley St headquarters in London with senior party figures he announced his decision.

No-one, it seemed, argued with his logic.

One senior party source told The Herald: "He told them he had decided to go. He decided to put the party's interests before his own. It was an honourable man doing the honourable thing. Effectively, it was a bloodless coup. It was not et tu Brute.' He fell on his own sword."

Sir Menzies's abrupt departure "with immediate effect" leaves Vince Cable, his deputy, in temporary charge of the party. Later today arrangements to elect a successor will be announced. Party sources pencilled in December 17 as the day when the new leader would be named.

In a short statement, Sir Menzies, who succeeded to the LibDem throne following the resignation of Charles Kennedy in January 2006, said: "It has become clear that following the Prime Minister's decision not to hold an election, questions about leadership are getting in the way of further progress by the party.

"Accordingly I now submit my resignation as leader with immediate effect. I do not intend to hold a press conference or to make any further comment."

The unexpected news broke just before 6.30pm when Simon Hughes, the party president, stood on the steps of the LibDems' HQ and told reporters: "Ming Campbell has this evening submitted his resignation as the leader of the Liberal Democrats with immediate effect.

"For the last two years Ming has given huge purpose and stability to our party. He has led the professionalisation of the party and he has led the very successful preparations for the General Election whenever it comes.

"Throughout all his political life Ming has taken all his decisions - and this decision is one of them - in the interests of our party and of liberal democracy throughout Great Britain and every Liberal Democrat and many other people owe him a huge debt of gratitude."

Mr Cable, the party's Treasury spokesman standing alongside Mr Hughes, said: "Ming has earned the gratitude, the respect and the affection of our party and that was reflected in the warmth that his speech was received at the party conference a few weeks ago.

"And in that speech he set out a key message - that we are the only party in Britain with a clear vision of a fairer and greener country. He has had a long political career and has fought through that career for social justice and liberal values. We will ensure that we build on that political legacy," he added.

Both men declined to answer questions and did not respond to questions from journalists which included: "Did you wield the dagger?" Later, Mr Cable, who is expected to stand in for Sir Menzies at the weekly Prime Minister's Questions, insisted he did not think Sir Menzies was pushed.

"He took a fresh look at where he stood, discussed this with his family and colleagues and decided the best thing he could do in the interests of the party was step aside." He added: "Politics is a very brutal business."

Minds will now turn to the succession. Nick Clegg, the 40-year-old Home Affairs spokesman, is odds-on favourite to become the new leader with his main rival almost certain to be Chris Huhne, 53, the Environment spokesman, who unsuccessfully bid for the leadership in 2006.

Other names might also be floated. Lembit Opik, the Welsh MP, replied with a "no comment" when he was asked if he would stand, and speculation will continue over whether Mr Kennedy, 47, the former leader and MP for Ross, Skye and Lochaber, should throw his hat into the ring.

Matters came to a head yesterday after Mr Cable admitted his leader's future was "under discussion". Lord Taverne, the LibDem peer, was less equivocal and declared that unless there was a change of leadership, then "the party goes down the drain".

Within hours the leader had gone.

Last night Sir Menzies was said to be travelling back to his home in Edinburgh. Tributes poured in from political foes.

Gordon Brown, the Prime Minister, said: "Sir Menzies Campbell is a man of great stature and integrity who has served his party and country with distinction. "His contribution on foreign policy and international affairs is valued throughout the world. I'm sure he will continue to make a major contribution to public life."

David Cameron, the Conservative leader, added his tribute, saying: "Sir Menzies Campbell has had a distinguished parliamentary career and is a fine public servant. I wish him and his wife Lady Campbell well for the future."