The depth of opposition among his own party to any change will be underlined today by the launch of a campaign to keep the first-past-the-post system. It has the backing of more than 100 Labour MPs, including some ministers.
Mr Blair told cabinet colleagues yesterday to avoid splits by uniting behind a call for a public debate on the Jenkins report. "The collective approach is there must be a debate, whatever people's different views are," said the Prime Minister's official spokesman.
"Nobody is going to shut their mouths, but equally nobody will be raising the banner for one side or the other. This is a united and disciplined government and the Cabinet will be disciplined and united behind the line."
Mr Blair gave the report by the commission under Lord Jenkins of Hillhead a warm welcome, but there were clear signs last night that the Government will postpone the referendum on the electoral reform proposals until after the election, in spite of its manifesto commitment.
The Jenkins report gave Mr Blair a let-out clause by making it clear that no change to the voting system could be implemented before the next election, and it could take as long as eight years. There was growing speculation that proposals could be combined with a plebiscite on long- term plans for an elected House of Lords.
The commission, as expected, recommends the first-past-the-post system of voting should be replaced by a system based on the "alternative vote" plus a top-up (AV-plus) of 15 to 20 per cent.
The Home Secretary, Jack Straw, an opponent of proportional representation, was noticeably cooler than Mr Blair in the official government response, pointing out the difficulty in getting legislation through before the election to carry out a referendum on changing the voting system.
The Leader of the House, Margaret Beckett, declared herself a "sceptic", while other cabinet opponents of PR, including John Prescott, the Deputy Prime Minister, are understood to be content with the report because it has been "kicked into the long grass".
Downing Street last night reacted quickly to quash suggestions that Mr Blair's response was negative. "There is long grass, and long grass. We are not kicking it off into the far distance. The Prime Minister is very serious when he gives it a warm welcome."
The tone of the Prime Minister's response was enough to protect Paddy Ashdown from any threat of an immediate challenge to his leadership.
The Liberal Democrat leader, who briefed his MPs on the eve of the report, has been in close contact with Mr Blair, and is convinced that the Prime Minister has moved from his previous position of being "not persuaded".
Mr Blair said the report made a "well-argued and powerful case" for reform. But in a clear signal that the referendum could be delayed, he said: "We must take account of the radical and ambitious programme of constitutional reform we have already put in place together with closely associated changes to come, particularly the reform of the House of Lords."
William Hague, the Tory leader, set the Conservatives firmly against change and promised Mr Blair "the battle of his life" if he attempted to do so.