A number of former Labour cabinet ministers have pledged to oppose reform of the Commons electoral system ahead of a referendum due in May.
John Prescott, David Blunkett, John Reid and Lord Falconer have signed up as patrons of the "No to AV" campaign - while Margaret Beckett will be its president.
They will join forces with a number of Conservative cabinet ministers, including justice secretary Ken Clarke, foreign secretary William Hague and education secretary Michael Gove.
The referendum was a key concession won by the Liberal Democrats as part of the coalition negotiations with the Conservatives.
While the Conservatives largely oppose replacing the current first-past-the-post system and the Lib Dems support it, Labour appears split on the issue.
Labour leader Ed Miliband says he will be backing a change to AV in the referendum.
But former foreign secretary Beckett said: "This is so important it has to rise above party politics.
"Only three other countries use AV and one, Fiji, is abandoning it. It led to a significant drop in the number of people voting in Australia - that's why they had to make voting compulsory. AV doesn't help democracy, it stands in its way."
Former deputy prime minister Lord Prescott attacked the referendum as a "shoddy little deal that the Lib Dems made with the Tories as their price for power".
And Lord Reid said there was "no credible intellectual or political case that can be made for AV".
A pro-AV campaign was launched in September at the Lib Dem conference, with deputy prime minister Nick Clegg saying the current voting system is "not fit for purpose".


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