LONDON - The Liberal Democrats began the search on Tuesday for their third leader in 18 months after Sir Menzies Campbell resigned in the wake of falling poll ratings and growing criticism of his style.

|PIC1|The 66-year-old Campbell had also been widely regarded as being too old to lead a successful recovery in a general election likely to be still two years away.

He quit on Monday evening, less than a month since vowing at the party conference in Brighton to use his wisdom and experience to guide his party to victory.

The two favourites to succeed him, Home Affairs Spokesman Nick Clegg and Environment Spokesman Chris Huhne, both said they would make announcements shortly about whether they would stand.

Leading party members said they had been surprised by the timing of Campbell's decision and denied reports he was the victim of a coup.

They said Campbell had been the victim of unfair and ageist criticism in the media but insisted he had chosen his own moment to stand down.

"It's very clear he made up his own mind when he was up in Scotland this weekend," acting party leader Vince Cable told Sky News.

"I and the vast majority of my colleagues in the House of Commons made it absolutely clear we were fully behind him.

"There is absolutely no question of plots."

But Liberal Democrat backbencher Mike Hancock said Campbell had been hounded out.

"We just didn't give the poor man a chance," he told the BBC.

He blamed "a right shower of people, who were stirring it behind his back, didn't have the guts to come forward and spell it out to his face or even to the parliamentary party.

"I think it's absolutely despicable."

Nominations open on Tuesday and close at the end of the month. Ballot papers will be sent to all party members from November 21 and must be returned by December 15, with the result announced two days later.

In a resignation letter, Campbell said his decision had been triggered by Prime Minister Gordon Brown opting this month not to call a snap election.

"It has become clear 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," said Campbell.

"Accordingly, I now submit my resignation as leader with immediate effect."

Campbell had been criticised for his lacklustre performance in parliament and mercilessly mocked in the press and by cartoonists for his age.

The party's support in opinion polls had halved since the 2005 general election to just 11 percent with its popularity steadily sliding against both Labour and the Conservatives.

Campbell took over the Liberal Democrats in March 2006 at a time when they were in disarray after the resignation of previous leader, Charles Kennedy, over an alcohol problem and a series of sexual scandals within the party.