Gordon Brown would consider surrendering the Prime Minister's traditional right to choose the timing of a general election in an attempt to secure a four-year agreement with the Liberal Democrats in the event of a hung parliament.
Labour is actively investigating the introduction of fixed-term parliaments, under which an election would be held every four years. It would remove the Prime Minister's right to ask the monarch for parliament to be dissolved – seen as one of the premier's most important powers, since it gives the governing party a huge advantage over the opposition. A switch to fixed-term parliaments is thought to enjoy the backing of a majority of the Cabinet. It would either be included as a pledge in Labour's election manifesto or offered as a concession to the Liberal Democrats if there is a hung parliament.
Nick Clegg's party, which has long supported the idea, is nervous about Mr Brown or David Cameron calling an early second election later this year if they fail to win an overall majority. A second poll within months could see the third party badly squeezed as voters made a final choice between Labour and the Conservatives.
The plan was revealed in an interview with The Independent by Peter Hain, the Welsh Secretary, who said: "Labour being prepared to deliver fixed-term parliaments as part of a negotiation to sustain a government in power is attractive, whether we have a majority or are just short of a majority." He said a four-year deal with the Liberal Democrats would provide stability and calm the financial markets, which are worried that painful decisions on public spending cuts would be put off if a second election were in prospect.
"What the country needs is some certainty in the aftermath of the banking crisis to secure recovery. A fixed term would also give the markets the stability and certainty they crave," he said.
Mr Hain said the plan would reassure the Liberal Democrats that "there would not be some immediate crisis or tactical pulling of the rug [to call another election] for party advantage".
Fixed-term parliaments would be part of a Labour drive to "transform the political landscape" which would appeal to "mainstream majority voters". Other elements included a fully-elected House of Lords and bringing in the alternative vote for Westminster elections, under which people rank candidates in order of preference so that the eventual winner enjoys majority support.
"Whatever the [election] result, this kind of politics has got to be the politics of the next Parliament. The current Parliament has been so badly discredited – not just because of MPs' expenses. The bottom has fallen out of trust in politics. Labour's radical democratic reforms are particularly appealing to Liberal Democrat supporters. The Tories will never offer them electoral reform," he said.
Mr Hain declined to comment on whether fixed-term parliaments would be included in Labour's manifesto. However, a Brown aide confirmed the idea is "in play". Other ministers said to support the proposal include Ed Miliband, the Energy and Climate Change Secretary, who is writing the manifesto; his brother David, the Foreign Secretary; Alan Johnson, the Home Secretary and John Denham, the Communities Secretary.
Asked whether the move was intended to encourage anti-Tory tactical voting or woo the Liberal Democrats in a hung parliament, Mr Hain replied: "Both". He claimed there was an "anti-Tory majority" in the country.
He added: "I am encouraging people to vote Labour in the marginals that will determine whether we have a Tory government bent on savage cuts and opposed to democratic reform, or a Labour government that would manage the economy in a way that protects public services whilst reducing the deficit and protecting jobs.
"The main thing is to try to assemble the mainstream decent majority that is increasingly terrified about the prospect of a Tory government."
Asked how he would urge people to vote in Conservative-Liberal Democrat marginals in the South-west, Mr Hain replied: "I am in the Labour Cabinet. My responsibility is to support all Labour candidates. My priority and the country's priority is the Labour-Conservative battleground. The Liberal Democrats are well dug in in the South-west. I leave my good friend Billy Bragg [the singer who campaigns for anti-Tory tactical voting in Dorset] and others to say what they have said in the past."
Mr Hain denied that Labour's move was designed to "hoover up" the votes of people who might be tempted to support the Liberal Democrats. "It is about the future of Britain... This is a bigger issue than party advantage. It is not a pre-election ploy. It would be a permanent commitment alongside House of Lords and electoral reform."
The Welsh Secretary believed many voters who had deserted Labour for other parties or planned not to vote were returning to the fold because they were "frightened" by the prospect of a Tory government. "They are starting to say 'We will think again', after pretty well writing Labour off. That means that the next election, as the polls are confirming, is wide open."
Denying his move was a tacit admission that Labour could not win an overall majority, he said: "There is every prospect of us having a majority. I don't think anybody expects us to have a large majority. The electorate will decide whether we have a majority or we are just short of a majority.
"We need this kind of programme in the next Parliament in any case. This is a politics whose time has come. Politics has been so badly discredited. The damage is cataclysmic. To think we can carry on as before is astonishingly complacent. If we don't do this, I think the next Parliament will get its comeuppance, just as the current Parliament has."
Privileges of power: How the law stands
Under the British constitution, sweeping executive powers, known as the Royal Prerogative, are officially vested in the monarch – including the power to summon, prorogue (adjourn) and dissolve Parliament. In exercising these powers, the sovereign normally defers to the advice of the Prime Minister or of other ministers.
A sitting Prime Minister can ask the monarch to dissolve Parliament at any time so that a general election can be held.
Britain is unusual among democracies in not having fixed-term parliaments. Westminster is isolated within the UK, since the Scottish Parliament, Welsh and Northern Irish Assemblies are elected for a four-year fixed term.
Parliament is dissolved after a maximum five-year term, although this can be overridden in a national emergency.
An election can be delayed until five years after the first day on which Parliament sat, rather than after the previous election. This means that Gordon Brown could delay this year's poll until 3 June.