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Five former Labour cabinet ministers have pledged their opposition to changing the system for electing MPs.

Margaret Beckett, David Blunkett, Lord Falconer, Lord Reid and Lord Prescott all said they would campaign against replacing first-past-the-post with the alternative vote (AV) system.

Both sides are mobilising support ahead of a referendum planned for 5 May 2011.

While the Conservatives largely oppose change and the Lib Dems support it, Labour appears divided over the issue.

Labour leader Ed Miliband has suggested he will campaign for a switch to AV but is opposed to holding the referendum on the Westminster voting system on 5 May, the same day as devolved elections in Scotland, Wales and Northern Ireland.

The Conservatives and Lib Dems agreed to hold a referendum as part of their coalition deal but Prime Minister David Cameron and his deputy Nick Clegg are on different sides of the argument over the need for change.

The No to AV campaign said a clutch of senior Labour and Conservative figures had agreed to act as "patrons" in its cross-party fight to retain the existing system, providing advice and acting as spokespeople.

Mrs Beckett, a former foreign secretary, is to become president of the campaign, while former Labour MP Joan Ryan will become its deputy director.

Mrs Beckett said AV had not proved successful when it had been introduced in other countries.

"This is so important it has to rise above party politics," she said. "AV does not help democracy. It stands in its way."

Pledging their support, former Home Secretary David Blunkett said the AV system was "unwanted, unfair and complicated" while Lord Reid said it would be a "backward step" for democracy.

"There is no credible intellectual or political case that can be made for AV," he said.

Among senior Conservative politicians to say they will become patrons of the campaign are Foreign Secretary William Hague, Justice Secretary Ken Clarke and party co-chairman Baroness Warsi.

"Ditching our current voting system in favour of the alternative vote would not be a sensible move," Mr Clarke said, arguing it could produce "unexpected and disproportional results".

At the Lib Dem party conference in September, leader Nick Clegg argued the current system was "not fit for purpose" and "a relic that deserves to be consigned to the past".

He insisted "fair votes" campaigners - many of whom want a proportional system but are backing AV as a "first step" towards further reform - could win a referendum.

Jonathan Bartley, from the campaign group Yes to Fairer Votes, suggested opponents to AV represented the political establishment fighting against a process preferred by ordinary people which would make it harder for them to get elected.

"Most of the people on this list grew up in the 1950s when we lived in a two-party system and when first-past-the-post arguably worked," he said.

"What they don't recognise is that the public has changed since then. People now vote for a variety of parties, on a variety of issues and our voting system needs to be upgraded to meet with where the public are."

Among Labour politicians thought to favour a switch to AV are shadow chancellor Alan Johnson and former Foreign Secretary David Miliband.

It is not clear what role the party leaders will play in the referendum campaign, with suggestions that the two camps want non-political figures to take the lead in trying to engage the public over the issue.

MPs have approved plans for the referendum date despite concerns that combining it with devolved elections will detract from these polls and potentially confuse voters.

Peers are currently debating the bill paving the way for the referendum.

Under the AV system, voters rank candidates in their constituency in order of preference.

Anyone getting more than 50% of first-preference votes is elected. If no-one gets 50% of votes, the candidate with the fewest votes is eliminated and their backers' second choices allocated to those remaining. This process continues until one candidate has at least 50% of all votes cast.