A senior Liberal Democrat Minister 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 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such as Chancellor George Osborne and Foreign Secretary William Hague would be undermined unless the allegations were withdrawn.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 said.
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, MP for Richmond, had earlier sought to play down the differences, insisting the coalition was working well, even though feeling were “running high” over the AV issue.
But 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 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.”
Earlier, Lib Dem deputy leader Simon Hughes had threatened to refer the No campaign to the Electoral Commission, but Mr Huhne’s intervention 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 expensive vote-counting machines, contributing to the £250m cost of adopting it.
He said that the allegation was “demonstrably untrue”.
Mr Hague, however, insisted it was a legitimate issue.
“There is no doubt that having a more complicated system costs more. That is a legitimate issue to raise in such a campaign,” he said.
Mr Clegg had earlier used a newspaper interview to launch his most outspoken attack on the No camp, denouncing it for running a campaign of “lies, misinformation and deceit”.
“This nasty No campaign, I hope, will prove to be the death rattle of a Right-wing elite, a Right-wing clique who want to keep things the way they are, “ he said.
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