A group of shadow cabinet ministers have said they will campaign for a change to the Westminster voting system at a referendum set for next May.
Alan Johnson, Sadiq Khan, Tessa Jowell and Douglas Alexander are among 50 Labour MPs to urge a switch from first-past-the-post to Alternative Vote (AV).
One of them, ex-minister Ben Bradshaw said AV would give voters "more power".
The "No" campaign says AV is "complex" and "untested". It has the backing of Tory ministers and Labour veterans.
Both sides are mobilising support ahead of next year's vote, which will take place on 5 May if legislation authorising the poll is approved by Parliament.
Electoral reform supporters launched the "Yes to AV" campaign last month and it has the backing of Nick Clegg and other senior Lib Dems - who are running their own separate "fair votes" push.
Labour supporters of AV - including frontbenchers Peter Hain, Hilary Benn, John Denham, Liam Byrne and former Business Secretary Lord Mandelson - have now launched their own movement for change, backed by groups such as Compass and Progress.
"We have a historic chance to reform our unfair and out of date voting system," Mr Bradshaw - a former culture secretary - said.
"Voting yes would lead to a small change that makes a big difference. AV gives more power to people, freeing them from the pressure to vote tactically and ensuring every vote counts. It would also mean MPs and candidates having to work harder to win the support of at least half their voters."
In the current first-past-the-post system, the candidate who secures the most votes is elected.
Under AV, voters would be able to rank candidates on the ballot paper in order of preference.
If no candidate wins more than 50% of the votes in the election, the candidate with the least support is eliminated and their second preference votes shared out among the remaining candidates.
This process continues until someone gets a majority of the votes.
'Tried and tested'
Labour has not said whether the party will campaign for a yes or no vote in the referendum although it included a pledge to give public a vote on the issue in its general election manifesto.
While Labour leader Ed Miliband has indicated he favours a switch to AV, he is unhappy that the poll is likely to be held on the same day as national elections in Scotland, Wales and Northern Ireland and has suggested the referendum will not be a priority for him.
Launching its own campaign last month, the No camp said it had the backing of Conservative Cabinet ministers William Hague and Ken Clarke as well as former senior Labour ministers Lord Prescott, Lord Reid and David Blunkett among others.
On Sunday, it unveiled the names of 20 newly-elected Conservative and Labour MPs which it said agreed the referendum was not a priority for people facing severe economic pressures.
"Next year the British people will have to make a decision on whether they want the country to start using a voting system which nobody really wants, which is complicated and unfair and which is massively expensive to administer - or to stick with the current tried and tested system that delivers clear, effective and decisive results," Labour MP Margaret Beckett - the campaign's president - said.
Prime Minister David Cameron says he wants to keep the existing system and will campaign for it.