MPs are to vote on Gordon Brown’s plan for a referendum on changing Britain’s "first past the post" voting system.
The prime minister has suggested it be replaced by an "alternative vote" system, in which voters rank candidates in order of preference.
Last week he said a referendum could be held by November 2011 and could help renew public faith in politics.
But with little time left before the general election the Tories accused him of trying to "fiddle" the system.
On Tuesday the government will table an amendment to the Constitutional Reform Bill for a referendum for an alternative vote system after the election.
‘Directly accountable’
Announcing the plan last week, Mr Brown said it was part of a wider commitment to constitutional reform.
He said the alternative vote would let MPs keep their link to their constituencies and be elected with much broader support than just from those who picked them as their first choice.
WHAT IS ALTERNATIVE VOTE- Voters rank candidates in order of preference and anyone getting more than 50% in the first round is elected.
- If that doesn’t happen, the candidate with the fewest votes is eliminated and their second choices allocated to the remaining candidates
- This process continues until a winner emerges
"In short it offers a system where the British people can, if they so choose, be more confident that their MP truly represents them, while at the same time remaining directly accountable to them," Mr Brown said.
But, with a general election looming, some were suspicious that it was an attempt to win over the Liberal Democrats – who have long campaigned for electoral change – in the event of a hung parliament.
Labour pledged a referendum on electoral reform in its 1997 election manifesto but the idea was kicked into the long grass by Tony Blair after his landslide victory.
‘Death-bed conversion’
Asked why he was looking at it now, Mr Brown said the damage done to Parliament’s reputation by the expenses scandal had demonstrated the need to act.
But Conservative leader David Cameron mocked him at last week’s prime minister’s questions, saying the last Lib Dem leader to be offered a deal on electoral reform was Paddy Ashdown.
He quoted Lord Ashdown’s diaries in which he noted former PM Tony Blair had backed the idea but told him: "I can’t get it past Gordon".
The Conservatives are strongly opposed to replacing the current "first past the post" system, saying it results in stable governments.
The Liberal Democrats intend to put forward their own amendment to the bill. The party says the AV option is "a small step in the right direction" but not a substitute for a fully fledged proportional system.
"If they agree it, this is a death-bed conversion from a party facing defeat at the general election," said home affairs spokesman Chris Huhne.
A number of Labour MPs are also thought to be sceptical of the idea and, with time for parliamentary business running out before the election, the measure stands little chance of becoming law.
As a result several would-be Labour rebels told the BBC they would not vote against the government, one declaring: "It’s dead before it’s even started – so what’s the point"
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