Although allies of Mr Ashdown and Tony Blair insisted last night the relationship would not be affected, MPs in both parties predicted that there would be a "distancing operation" after the Liberal Democrat leader stands down in June.
Relations with Labour are bound to be a critical issue in the leadership contest to choose Mr Ashdown's successor. His policy of "constructive opposition" to Labour is unpopular with many grassroots Liberal Democrats, and so leadership candidates will be tempted to adopt a more cautious policy.
Much of the closer union between the two parties was down to the strong personal bond between Mr Blair and Mr Ashdown, who met regularly in the run-up to the 1997 general election - often as a foursome with their wives.
Labour's landslide victory was bitter-sweet for the Liberal Democrat leader. Although he doubled his number of Westminster troops to 46, he knew that his preferred outcome of a hung Parliament - and the prospect of a Lib-Lab coalition, which he had discussed privately with Mr Blair - would elude him during his spell as party leader.
But to Mr Ashdown's amazement, Mr Blair found time to telephone him on the day after his great victory. "The deal is still on," the Prime Minister told him.
The "deal" meant that, although there would not be a formal coalition, Mr Blair would work closely with the Liberal Democrats. After talks involving Peter Mandelson, then minister without portfolio, senior Liberal Democrats including Mr Ashdown were invited to join a new cabinet committee responsible for discussing constitutional reform.
Despite some grumblings in his party, Mr Ashdown could point to real gains from this co-operation, as the Government agreed to incorporate the European Convention on Human Rights into UK law and to bring in proportional representation for this June's European Parliament elections - a fitting swansong for Mr Ashdown.
Mr Blair was keen to forge even closer links, in the hope of forming a centre-left alliance that would dominate the next century in the way the Tories had dominated the 20th century.
Last November, the two leaders secretly agreed to extend the remit of the cabinet committee to other policy issues. But both ran into immediate flak from their own parties, and Liberal Democrat MPs demanded a veto on which policy areas could be discussed.
Mr Ashdown was known to be angry at the caution of his troops, but came under further pressure because of Mr Blair's refusal to commit himself to a referendum in this Parliament on electoral reform for the House of Commons.
Undaunted - and knowing he would stand down - Mr Ashdown managed one last advance this month when he and Mr Blair agreed the cabinet committee would discuss plans for a new European Union foreign and defence policy.
Labour critics of Lib-Lab co-operation, led by John Prescott, the Deputy Prime Minister, used Mr Mandelson's resignation last month as a platform to warn Mr Blair not to push ahead with closer collaboration. They will be delighted that Mr Ashdown's decision to quit will boost their campaign.
But Mr Blair will not be deflected from his historic mission. Whoever succeeds Mr Ashdown will receive the full force of the Prime Minister's charms. "We may have a temporary chill, but in the long run Tony Blair will ensure that warm relations resume," one Blairite insisted last night.Reuse content