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From BBC News article: http://news.bbc.co.uk/1/hi/uk_politics/7045833.stm

Sir Menzies Campbell has resigned as leader of the Liberal Democrats "with immediate effect".

Senior Lib Dems Vincent Cable and Simon Hughes made the announcement, saying the party owed Sir Menzies "a huge debt of gratitude".

Deputy leader Mr Cable will take over as acting leader for now - a leadership timetable will be announced on Tuesday.

Mr Hughes said Sir Menzies had taken the decision in the "interests of the party and of Liberal Democracy".

Sir Menzies was not present when the announcement was made.

The BBC understands that, in a private meeting with Lib Dem members, he said that he did not believe he could turn around his and the party's current poor poll ratings.

He indicated that now it was clear that there would be no election for many months he wanted to give his successor time to bed in before the election.

Party president Mr Hughes said Sir Menzies had brought "purpose and stability" to the party since he took over as leader, after Charles Kennedy quit in January 2006.

He added that he had put in place the preparations for the next general election "whenever it comes".

Speculation has been growing about the Lib Dem leadership, particularly since Prime Minister Gordon Brown decided not to call an early general election - and indicated he may not do so until 2009 at the earliest.

But a spokeswoman for Sir Menzies said he had made the decision to stand down himself and had handed his resignation letter to the party's president on Monday afternoon.

She said he had stood down in the interests of the party because he did not want the Liberal Democrat message drowned out by media speculation about his future.

Earlier Mr Cable had conceded the party's leadership was "under discussion" but said he did not believe Sir Menzies' job was under threat.

Speaking to the BBC later he said he had not been aware of the "imminence of the decision".

"I'm very sad he felt he needed to step aside," he said, but added: "I don't think he was pushed."

"There was a very open debate about this immediately that Gordon Brown made his decision to postpone the election.

"I think he took a fresh look at where he stood ... and decided the best thing he could do in the interests of the party was to step aside."

He said he thought there would be a leadership election before the new year.

The prime minister has paid tribute to Sir Menzies as a "man of great stature and integrity who has served his party and country with distinction".

And Tory leader David Cameron said he was a "fine public servant" who had had a distinguished parliamentary career, and wished him well for the future.