Ministers are preparing to force a parliamentary debate within months on scrapping the first-past-the-post voting system in an attempt to spell out dividing lines on the issue with the Tories.
A cabinet committee agreed to press ahead with plans to offer a referendum on electoral reform in Labour's election manifesto, Whitehall sources confirmed. Ministers have accepted that time has run out to stage the ballot alongside the election, expected in May, but they are ready to try to push the issue on to the Commons agenda early next year.
Such a move, they say, could wrong-foot David Cameron's Conservatives by portraying the party as defenders of a discredited voting system. They are considering publishing a "paving bill" which would commit the next government to staging a referendum.
Alternatively, they could attach an amendment on a referendum to the Constitutional Reform Bill announced in last month's Queen's Speech. Either tactic would force the Tories, who are staunchly committed to the first-past-the-post system, to spell out their position on electoral reform and then block any change.
Most ministers now back a referendum, with Alan Johnson, the Home Secretary, Ben Bradshaw, the Culture Secretary, and Tessa Jowell, the Cabinet Office Minister, originally arguing it could be held on election day. A final decision on tactics is likely to be taken at next week's meeting of the full Cabinet. Some ministers even argue that putting electoral reform on to the agenda now could help negotiations with the Liberal Democrats in the event of a hung parliament.
The referendum would offer a choice between the current system and the alternative vote, in which voters rank candidates in order of preference. The second choices of the last-placed candidate are redistributed until someone receives 50 per cent of the votes cast.
The moves come two months after Gordon Brown announced plans for a referendum at Labour's annual conference. He told activists: "There is now a stronger case than ever that MPs should be elected with the support of more than half their voters."
Some critics believe Labour should be pressing for a purer form of proportional representation, such as the single transferable vote. Ken Ritchie, chief executive of the Electoral Reform Society, said: "We will be delighted if the Government gets around to backing voting reform before it is too late for it to do so, but we are not opening the champagne just yet."