The Liberal Democrat leader dismissed as "nonsense" suggestions that it amounted to the first step towards a Lib-Lab coalition. But the joint declaration underlined the close relationship between the two leaders, in spite of their regular clashes at Prime Minister's Question Time.
Robert Maclennan, who set up the liaison with Labour through Robin Cook - the strongest supporter of proportional representation (PR) in the Cabinet - said the statement was timed to show doubters that the two sides to the joint cabinet committee on constitutional reform had not run out of steam.
Mr Ashdown said that it was "almost unimaginable" to contemplate being offered a cabinet seat given Labour's overwhelming majority. But there are strong suspicions at Westminster that the development of a close working relationship through a cabinet committee could lead to a formal pact in the next Parliament.
The joint declaration sets out a broad framework of action, including the work by the Jenkins commission for a reformed voting system for the Commons reflecting "broad proportionality".
Although the declaration does not commit either party to PR for the next election, Mr Ashdown made it clear that he wanted to "prod" the Government to accept PR, provided there was a "yes" vote in the referendum before the next election. The declaration offers a united front to the Tories on Mr Blair's plans for reform of the House of Lords in the next session of Parliament, the review of the voting system, freedom of information, and modernisation of the way Britain is governed.Reuse content