Liberal Democrat leader Sir Menzies Campbell says he believes there is a one-in-six chance of a general election being held in October this year.
Sir Menzies Campbell has told his party to 'hold their nerve'
He said his party was on standby for a snap poll, although he also estimated a 35% chance of a vote being held in May 2008 - with a 50% chance of May 2009.
He said the PM, currently enjoying a "bounce" which would not last, might try to pull "a rabbit out of the hat".
Sir Menzies said his message to Lib Dem members was to "hold your nerve".
He said he did not expect there to be a hung parliament, telling journalists the British public always tended to go one way or the other.
The last general election was held in 2005, meaning Labour does not have to hold an election until May 2010 - although May 2009 has long been the favourite date.
On other matters he said the Lib Dem party conference, which like Labour's is in September, would focus on "solid policies" such as the environment and climate change.