Image caption The referendum date coincides with various elections

A referendum on changing the UK's voting system is planned for 5 May 2011, the BBC understands.

A vote on changing from first-past-the-post to the alternative vote (AV) could be held on the day of Holyrood, Welsh Assembly and English local elections.

But the government must first get it through Parliament and may face a rebellion by Tory MPs.

Many Labour MPs oppose a referendum - something pledged as part of the Tory-Lib Dem coalition deal.

It is thought the referendum will be linked to plans to change constituency boundaries - a key concern of the Conservatives but something fiercely opposed by Labour.

'Unfair'

The Tories pledged to make constituency sizes more "equal" in their manifesto - Labour say the plans are designed to eliminate smaller inner-city seats which they hold.

The proposed date of the referendum is due to be confirmed at a cabinet meeting next Tuesday and is likely to be announced by Deputy Prime Minister Nick Clegg later that day.

But legislation will not be debated until the autumn and may face a sizeable rebellion. Under the coalition deal, Tory MPs will be whipped to support a referendum - but will then be free to campaign against a change.

ANALYSIS It may seem odd that the date of 5 May should be so crucial - but for Nick Clegg it is a major victory. It significantly strengthens his own position and the prospects for the coalition holding together. The great fear in Lib Dem circles was that the AV referendum would be booted off into the long grass by a Conservative Party that regards voting reform as an unwelcome distraction. 5 May gives Lib Dems certainty - a definite time scale for achieving what most regard as the single most important concession wrung out of David Cameron in the coalition agreement. The hope too is that, because 5 May coincides with local, Scottish and Welsh elections, it will boost the chances of a "yes" vote by bolstering turnout. It also gives Nick Clegg the opportunity to campaign against the Conservatives and so underline his party's independence and demonstrate to anxious supporters that it has not become an annexe of the Conservative Party. Send us your comments

The Tories say the existing first-past-the-post system guarantees strong, stable government, while Lib Dems argue it is inherently unfair on smaller parties and allows candidates to be elected despite getting the support of only a minority of constituents.

Under the proposed AV system, voters rank candidates in order of preference and anyone getting more than 50% in the first round is elected.

If that does not happen, the candidate with the fewest votes is eliminated and voters' second choices allocated to the remaining candidates.

This process continues until a winner emerges.

A government source said holding the referendum on the same day as the elections should improve voter turnout.

The Liberal Democrats are set to campaign for the new system, while the Conservatives will oppose it.

The BBC's deputy political editor James Landale said the outcome could determine how long the coalition survived.

For many Lib Dems, it was the one reason they agreed to take part, he said, and if they lost, it could be a reason to leave.

Conservative MP Daniel Kawczynski, chairman of the all-party group for the promotion of first-past-the-post, said he was "very concerned" about AV and he would campaign "vigorously" against it.

Under AV, he said: "There are millions of Britons out there, like me, who have one preference - in my case it is the Conservative Party. We will be voting once, and yet other people will be given a second vote, and that is creating two classes of voters which is simply unacceptable."

But campaign group the Electoral Reform Society welcomed the news. A spokesman said: "This is a fantastic opportunity to turn the page on the old politics and an old system that's served it so well. This is a real chance for change."

'Peripheral issue'

The previous Labour government also promised a referendum on the AV system, which it planned to hold in autumn 2011.

But Labour may choose to give its MPs - many of whom oppose changing the voting system - a free vote on the matter.

Shadow foreign secretary David Miliband told BBC Radio 4's Today programme that if he wins the Labour leadership contest, he will back a referendum and allow activists to campaign in favour of a switch to AV.

He said: "I think it's important that we move to a system where every Member of Parliament has at least 50% of the vote of their constituents. At the moment only a third of MPs have that 50% backing.

"Alternative vote, which allows you to rank your preferences... allows that to happen."

His brother Ed, another leadership candidate, also said he would support changing the voting system.

He said: "I strongly support the case for introducing the Alternative Vote, to ensure greater fairness for voters and greater legitimacy for our MPs in Westminster. Whenever the referendum takes place, I will campaign with other supporters across the political spectrum for this important change."

But another leadership candidate, shadow health secretary Andy Burnham, told the Guardian that voting reform was "irrelevant" and "a peripheral issue".

"It is not my party's job to prop up the Liberal Democrats by helping them win a referendum that is important to them," he told the newspaper.