The leader of the Liberal Democrats has resigned.

The party’s President Simon Hughes announced this evening that Sir Menzies Campbell is stepping down immediately.

He has led the party since March 2006, when long-serving leader and fellow Scot Charles Kennedy resigned over press reports of about alcoholism.

Sir Ming’s decision to stand down was influenced by the Prime Minister’s announcement last week that he will not be calling a general election until 2009.

In a letter of resignation he said:

“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.”

The North East Fife MP has faced criticism over his age – he is 66, compared to the 41-year old Tory leader David Cameron.

Prime Minister Gordon Brown is 56.

An election will be held to replace Sir Ming on December 17th.

The party’s deputy leader and Treasury spokesman Vince Cable will act as interim leader.

A detailed election timetable will be announced tomorrow.

The Liberal Democrats are likely to pick Nick Clegg as their next leader. He is currently the party’s Home Affairs spokesman.

Since winning Sheffield Hallam in the 2005 general election, the 40-year-old has made a major impression on the Liberal Democrats in parliament and across the country.

A former MEP for the East Midlands from 1999 to 2004, he speaks five languages.

Another contender for the leadership is Chris Huhne, 53, a former City economist who is the party’s Environment spokesman. He finished second to Sir Ming in 2006.

It is not thought that veteran MP and current party President Simon Hughes will contend the election.

His campaign for leader in 2006 was damaged by revelations that he had regularly called a gay chatline.

At the Lib Dem party conference last month Sir Ming was insisting he would lead his party into and beyond the next election.

However, at that time an election was seen as likely to be in May 2008 or even October or November this year.

Low poll ratings for the party and the realisation that David Cameron is likely to take votes from the Lib Dems as much as Labour may have influenced his decision.

In March Sir Ming gave an interview to PinkNews.co.uk in which he pledged his support for gay rights.

He spoke at the Stonewall Absolutely Equal event at the Lib Dem party conference last month.