Image caption The battle between the Yes and No to AV campaigns has become increasingly hard fought

David Cameron and Ed Miliband have both joined forces with political opponents as the battle over the way Britain elects its MPs stepped up a gear.

Mr Cameron shared a platform with Labour former home secretary John Reid to make the case for keeping the current first-past-the-post system.

At a rival event, Labour leader Mr Miliband joined forces with Lib Dem Vince Cable to argue for a change.

Voters decide on 5 May whether to adopt the alternative vote system.

The battle between the Yes and No to AV campaigns has become increasingly bitter and hard-fought in recent weeks - as some polls suggest the result could be a close call.

'Poster boy'

It has spawned strange political alliances, with Mr Cameron and former home secretary Lord Reid - used to being on opposite sides of the Commons chamber during Labour's years in power - now swapping pleasantries on a joint platform.

Ed Miliband has also reached out across the party divide - but he faced questions about why he had chosen to share a platform with business secretary Vince Cable, after earlier refusing to campaign alongside Mr Cable's Lib Dem colleague Nick Clegg.

Media playback is unsupported on your device Media caption AV a 'self-interested outrage' say David Cameron and former Labour minister Lord Reid

He said: "I will share a platform with anybody I think can help us win a referendum.

"The No campaign want to make Nick Clegg the poster boy for this campaign, and what I'm saying is, 'Don't make this a referendum on David Cameron or Nick Clegg or Ed Miliband - make it a referendum about what kind of politics we want in this country.'"

Mr Miliband has said Mr Clegg's involvement would damage the vote-change campaign, but he urged his supporters not to vote against AV simply to "give Nick Clegg a kicking".

Mr Cable, who has found himself at odds with his Conservative coalition colleagues in recent days over immigration policy, denied that Mr Clegg was a "liability" to the campaign.

He said he "would like to see" Mr Miliband and Mr Clegg working together for a Yes vote - and insisted Mr Clegg would continue to lead the Lib Dems, with his support, if the Yes campaign failed.

Media playback is unsupported on your device Media caption Alternative vote 'fairer' say Labour leader Ed Miliband and Liberal Democrat Vince Cable

David Cameron, at his event, was also quizzed about attacks on Deputy Prime Minister Nick Clegg by the No campaign - in one campaign poster they warned the alternative vote would lead to "president Clegg".

Mr Cameron said he did not "condone any personal attacks" and his own Conservative No campaign literature did not feature any.

But Lord Reid seized on the spat between Mr Miliband and Mr Clegg, claiming that it was the Yes campaign's "biggest handicap".

"We are prepared to share platforms. I don't think the Yes campaign are, are they?" he added.

In his speech, Mr Cameron said that he and Lord Reid "don't agree on much" - except that the UK should keep its existing voting system.

He wasn't there this morning. Not at the Yes or No event. Yet the shadow of Nick Clegg could be felt at today's battle of the political odd couples Nick Robinson, BBC Political Editor Read Nick's thoughts in full

Lord Reid said there were some issues so important that they "transcend party politics".

Mr Cameron said one of his biggest objections to the alternative-vote (AV) system was that it would result in more coalition governments, and despite the current one being necessary "at a time of national need", that would tend to reduce political accountability.

"I can absolutely put my hand on my heart and say in preparing our manifesto we really did go through every pledge and thought, 'We could be accountable for this. We aim to have, and believe we can have, a single party government so don't put anything in your manifesto you don't believe you can deliver,'" he said.

"If you move to a system where coalitions become the norm rather than the exception I think you might find politicians start being very casual about what they put in their manifesto because you can put in policies that you know you can get rid of as you form a coalition."

'A diversion'

In his speech, Ed Miliband said that while AV was not a panacea, he believed it would improve politics by making MPs reach out beyond their traditional support bases to achieve at least 50% of the vote in their constituencies.

He said it would also encourage MPs from different parties to "seek points of agreement" rather than focusing on "how much they loathe each other".

As well as Mr Cable, Mr Miliband was joined by Alan Johnson, who resigned as shadow chancellor earlier this year.

Afterwards, Mr Johnson told the BBC his party leader was "absolutely right" not to campaign alongside Mr Clegg.

The referendum choice At the moment MPs are elected by the first-past-the-post system, where the candidate getting the most votes in a constituency is elected. On 5 May all registered UK voters will be able to vote Yes or No on whether to change the way MPs are elected to the Alternative Vote system. Under the Alternative Vote 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 in that round. Analysis: Foes become friends Q&A: Alternative vote referendum AV poll: Where parties stand

"In a sense, Nick Clegg should be supporting the argument that he stays off the platform because we want to win as many people over as possible and Nick Clegg is becoming a diversion," he said.

But the deputy prime minister said he did not believe people would be swayed, when it came to voting, by which politicians shared platforms during the campaign.

"I get the sense that, finally over the last 24 hours, the argument is being had on its merits," he told the BBC. "I think, after the expenses scandal, people want to clean up politics..I believe, as does Ed Miliband and many other people, that it would be better to do that through the new AV system."

The Conservatives agreed to the referendum as part of the coalition deal, which also allows both parties to campaign on opposing sides - but have insisted that the coalition will continue whatever the result.

Labour is split over AV - Mr Miliband is campaigning to change the system but other senior Labour figures, including former deputy PM Lord Prescott, want to keep first-past-the-post.

Under the first-past-the-post system voters put a cross next to their preferred candidate while with AV voters rank candidates in order of preference.

These preferences could be used to decide the outcome in places where no candidate wins more than 50% of votes cast.

Research published on Monday suggested support for retaining first-past-the-post may be increasing. An ICM poll for the Guardian gave the No vote an 11-point lead over its rival, compared with a two-point deficit in February.

Among those who said they had definitely made up their mind, the No camp's lead was 16 points. But 23% of those canvassed said they had yet to make up their mind.