The BBC's Caroline Quinn: The frontrunner now looks likely to be Charles Kennedy

The 58-year-old MP had previously been let it be known that he was considering standing as a candidate to succeed Paddy Ashdown - who steps down after the European elections on 10 June.

But Mr Campbell, the party's foreign affairs spokesman, has now decided after consulting with family and friends that he does not want to enter a contest he cannot be sure of winning.

His decision comes only weeks after another leadership hopeful Nick Harvey announced he was pulling out of the race in order to support Mr Campbell's candidacy.

Nicholas Jones reports: He was one of those who'd let it be known that he would like to be a contender

Mr Campbell, in a letter to his constituency chairman, said: "I have reached the conclusion that I should not put my hat into the ring.

"I have decided to make my intentions clear now to prevent further speculation."

He added that he would "continue to represent North East Fife as an MP and to support and campaign for the Liberal Democrats to the best of my ability".

Mr Campbell's exit reduces the field of potential candidates to about six.

The remaining front-runners include Simon Hughes, Charles Kennedy and Jackie Ballard the only women thought to be considering running.