A senior Liberal Democrat minister is threatening legal action against the No to AV campaign in a move which could potentially open up a damaging new rift in the coalition.
Chris Huhne, the Energy Secretary, said he is prepared to go to court over what he said were false claims made by senior Tories, including David Cameron, over the cost of introducing the alternative vote (AV) system for Westminster elections.
He warned it could be difficult for ministers from the two parties to carry on working together unless the Conservatives concerned - including Chancellor George Osborne and Foreign Secretary William Hague - withdrew the allegations.
"It is frankly worrying if you have colleagues, who you have respected and who you have worked well with, who are making claims which have no foundation in truth whatsoever. If they don't come clean on this I am sure the law courts will," he said.
Senior Conservatives sought to play down the divisions, with Mr Hague insisting that the coalition was working well, even though feelings on the AV issue - which had pitted the two parties against each other - were "running high".
But with another 10 days to go until voting in the AV referendum - which the Lib Dems demanded as the price of joining the coalition - the cohesion of the alliance between the parties is likely to be tested to the limits.
Both Mr Cameron and Deputy Prime Minister Nick Clegg are desperate to deliver a morale boosting victory for their respective supporters.
Mr Clegg used a weekend newspaper interview to launch his most direct attack yet on the Prime Minister, accusing him of aligning himself with the far right BNP and the Communist Party "in defence of the indefensible".
Lib Dem deputy leader Simon Hughes has also threatened to refer the No campaign to the Electoral Commission, but Mr Huhne's intervention has raised the prospect of a far more prolonged and damaging court battle.
Mr Huhne particularly picked on a claim by the No campaign that the introduction of AV would require the purchase of expensive vote-counting machines - an allegation which, he said, was "demonstrably untrue".
Mr Hague, however insisted it was a "legitimate issue" to raise the costs of introducing AV, which the No campaign has put a £250 million price tag on.
The Energy Secretary has been a particularly vociferous critic of the No campaign, having previously likened the tactics of Conservative Party chairman Baroness Warsi to those of Nazi propaganda chief Josef Goebbels.
It emerged at the weekend that he had now written to Mr Osborne accusing him of a "persistent resort to falsehoods" over the costs of introducing AV.
Despite the anger at the No campaign tactics, there were signs that some Lib Dems welcomed the chance for a public row with the Tories amid concerns among activists that Mr Clegg has become too close to Mr Cameron.
"The really good thing about this campaign is that it has made sure that Liberal Democrats stand up for what we believe in and nobody will think we are Tories any more," Mr Hughes said.
"Nobody can say in the future we and the Tories are the same."
Former Lib Dem leader Sir Menzies Campbell played down the damage from the friction, pointing out that Labour was just as divided on the issue of AV.
He also hinted that it could be helpful for the coalition parties to emphasise their differences.
"Of course we also know there are quite a few Conservatives not fully reconciled to the coalition," he told BBC Radio 4's Today programme.
"Perhaps they will be satisfied by the notion that David Cameron is not rolling over in front of Nick Clegg."
Former Tory leader Lord Howard said there was a "very important issue at stake" in the referendum, and both coalition partners had "different views".
"The truth is that these are two parties with different traditions and different views on many issues, which have come together to work together for the good of the country," he said.
"The need for them to work together in the good of the country is going to be just as great on May 6."
A senior Conservative backbencher hit out at the Deputy Prime Minister and Energy Secretary for "whingeing".
Mark Pritchard, Secretary of the Tory 1922 Committee, said: "Nick Clegg and Chris Huhne should stop their whingeing.
"With each of them presiding over major government departments they've never had it so good.
"Their personal and political sacrifices are infinitesimal compared to those made by the hundreds of public sector workers losing their jobs each week and many Conservative colleagues who gave up ministerial office for the sake of the coalition."