A challenge to Mr Kennedy is thought to be unlikely

In future candidates will need the support of "at least 10%" of its 62 MPs, whereas before they required the backing of just two.

The vote at the annual conference in Blackpool comes amid criticism of Charles Kennedy's leadership.

But Lord Kirkwood, who proposed the motion, said it was simply a "tidying up exercise".

He added: "There's nothing in this motion whatsoever with any events at the conference this week.

"It looks forward and has nothing to do with the leadership of this party as it stands."

But Colin Rosenstiel, a councillor from Cambridge, said: "I am against this motion because we are not the Tory party and our members can be trusted to make the right choice of leader when there is a contest."

Any leadership challenger must also have the support of 200 party members, spread across at least 20 constituency associations.

This policy does not change following the vote.

Ben Ramm, editor of The Liberal magazine, said the old rules on MPs' backing were fine and had "never been used flippantly".

However, the motion, which needed a two-thirds majority of delegates, passed comfortably.