Sir Menzies Campbell abruptly resigned as Liberal Democrat leader tonight in the face of mounting criticism of his leadership.

His departure, only 18 months after he took over from Charles Kennedy, was announced tonight by Simon Hughes, the party president, who said that the 66-year-old Scot had decided to step down "in the interests" of his party.

It opens up the leadership to a younger candidate with Nick Clegg, the home affairs spokesman, or Chris Huhne, the environment spokesman, the obvious candidates to lead the party towards the next election.

Speaking to reporters outside Lib Dem headquarters, Mr Hughes said: "Ming Campbell has this evening submitted his resignation as leader of the Liberal Democrats with immediate effect."

He said that Sir Menzies had brought "huge purpose and stability" to the party and helped make the party more professional.

He added: "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. Every Liberal Democrat and many other people who owe him a great debt of gratitude."

Vince Cable, the deputy leader, takes over as interim leader with immediate effect. The timetable for the election of a new leader will be announced tomorrow.

Mr Cable told reporters that Sir Menzies had "earned the gratitude, respect and affection of our party".

Sir Menzies did not attend his own resignation announcement but later released the text of his resignation letter.

He wrote: “When I was elected Leader of the Party in March 2006 I had three objectives. First, to restore stability and purpose in the party following my predecessor’s resignation and the leadership campaign itself, second to make the internal operations of the party more professional and third to prepare the party for a General Election.

"With the help of others, I believe that I have fulfilled these objectives, although I am convinced that the internal structures of the party need radical revision if we are to compete effectively against Labour and the Conservatives.

"But 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.

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