Mr Ashdown, who stands down on Monday after 11 years as leader, would have preferred Menzies Campbell, the Liberal Democrats' foreign affairs spokesman, or Nick Harvey, their campaigns chief. But neither man entered the race to succeed him.
Last night Mr Ashdown's office refused to reveal how he had voted, but close friends said he had opted for Mr Kennedy as the candidate most likely to continue his policy of forging close co-operation with Labour.
The four other candidates have opposed any extension of the remit of the cabinet committee which includes senior Liberal Democrat figures, forcing Mr Kennedy to harden his line during the leadership campaign. "Paddy has reluctantly come round to the view that it has to be Charles," said one friend.
Mr Ashdown has not been regarded as a member of the Kennedy fan club. Viewing him as too disorganised for a frontline post, Mr Ashdown made him agriculture spokesman and barked: "Where's Kennedy?" when he failed to turn up to meetings with military precision.
Mr Ashdown was irritated when Mr Kennedy appeared to distance himself last year from the strategy on co-operation with Labour. He believed he was playing to the party gallery ahead of a future leadership bid.
There was even speculation that Mr Ashdown ensured a marathon six-month leadership race in the hope that an "ABC" candidate - "anyone but Charles" - would emerge.
Although Mr Ashdown has remained neutral during the campaign, his decision could swing some last-minute support behind Mr Kennedy. The Liberal Democrats' 90,000 members have until tomorrow to return their ballot papers.
Mr Kennedy, the hot favourite when Mr Ashdown resigned in January, now faces a strong challenge from Simon Hughes, the party's health spokesman. At the outset, Mr Kennedy had hoped to win a clear mandate by landing more than half the votes cast, giving him a first round victory. But Mr Hughes is thought to have won enough support to deprive him of that.
The last of the five candidates would then drop out, with the second preferences of the people who supported him re-allocated among the other four. Insiders believe that Mr Hughes could pick up twice as many second preference votes as Mr Kennedy, who would therefore need a lead of about 12 or 13 points in the first round to avoid being overtaken in the later stages. "If Charles is only 10 points ahead, it will be a nail-biter," said one source.Reuse content