A senior Liberal Democrat minister has threatened legal action against the No to AV campaign as the battle over voting reform for Westminster elections descended into new levels of acrimony.
Chris Huhne, the Energy Secretary, accused senior Conservatives, including David Cameron, of repeatedly making false claims about the costs of introducing the alternative vote (AV) system.
He warned that the credibility of the Prime Minister and senior colleagues such as Chancellor George Osborne and Foreign Secretary William Hague would be undermined unless the allegations were withdrawn.
"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 told the BBC.
His incendiary intervention threatened to rock the cohesion of the coalition after Deputy Prime Minister Nick Clegg had for the first time attacked Mr Cameron directly, accusing him of "defending the indefensible".
With the polls pointing to a No vote in the referendum - now less than two weeks away on May 5 - there is deepening anger among Lib Dems, who strongly support AV, at the tactics of the No campaign.
Mr Hague had earlier sought to play down the differences, insisting that the coalition was working well, even though feeling were "running high" over the AV issue.
However, Mr Huhne warned that trust between coalition ministers could be irreparably damaged, making it far harder for them to work together effectively in the future.
"It is going to undermine the credibility of colleague ministers - the Prime Minister and the Chancellor of the Exchequer and the Foreign Secretary - if they use repeatedly allegations that have no foundation in truth whatsoever," he said.
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"That is not good for the coalition. We have a job to do in the coalition Government to clean up the mess we have inherited at the time of the last election. It is going to be a lot more difficult if you don't have the same respect for colleagues because, frankly, they have departed so far from the foundations of truth in an election campaign."
