Nick Clegg has laid bare a major coalition rift by revealing that he wants David Cameron to launch a sleaze probe into Culture Secretary Jeremy Hunt's handling of the BSkyB bid.
The Liberal Democrat leader has repeatedly told Cameron that the independent adviser on ministerial interests, Sir Alex Allan, should be brought in to consider the case.
The behind-the-scenes clash emerged as Clegg signalled that his MPs will abstain in a vote being forced by Labour on the issue on Wednesday.
The Commons motion calls for Sir Alex to investigate whether Tory Mr Hunt misled Parliament and failed to take responsibility for his special adviser Adam Smith, who resigned after admitting his contacts with News Corporation had been too close.
The Lib Dem decision was said to have received "unanimous support" at a meeting of the parliamentary party last night.
Clegg gave the news to Cameron when they met earlier, according to a senior party source.
"I don't think the Prime Minister was pleased about it," the source said. "The decision not to refer it to Sir Alex Allan was the Prime Minister's decision, and we respect that.
"However, it is not a decision that is endorsed by the Liberal Democrats, therefore we don't think we need to endorse it, therefore we won't support it on the floor of the House."
The source said Clegg had discussed the matter with Cameron on a number of occasions over recent weeks, and told him the Culture Secretary should be subject to a probe.
There were "clearly still questions remaining" after Hunt's evidence to the Leveson inquiry last month - despite Cameron's insistence that the issue had been settled.
Evidence of the coalition split will intensify pressure on the prime minister to call in Sir Alex for an independent assessment of whether the ministerial code was breached.
He is already facing accusations of double standards after triggering an investigation into Tory chairman Baroness Warsi, who has admitted failing to declare business links with a relative who accompanied her on an official trip to Pakistan.
Labour leader Ed Miliband could seek to capitalise on the situation when the two men lock horns at Prime Minister's Questions this lunchtime.
A senior aide to Cameron played down tensions with the Lib Dems, saying their stance on the Opposition Day debate was "not unexpected".
"It's a party political motion not government business," the aide added.
Senior Liberal Democrat Lord Oakeshott claimed on Monday evening Hunt should have resigned over the way he handled the BSkyB bid, telling Channel 4 "no self-respecting minister could possibly carry on."
Clegg is due to give evidence to the Leveson inquiry later, where he is expected to renew his criticism of Cameron and other politicians for getting too close to Rupert Murdoch's empire.
He could also face questions about the handling of the BSkyB bid, and the Prime Minister's appointment of former News of the World editor Andy Coulson as Downing Street communications director.
Clegg will be followed into the witness box by Scotland's First Minister Alex Salmond, who has been under growing pressure about his links to Murdoch.
Salmond has been accused of courting the media tycoon in order to win The Sun's support for the SNP - although he insists he was merely interested in safeguarding Scottish jobs.
The inquiry could also seek to learn whether the SNP leader was among those who had their phone hacked. He has previously refused to confirm if police have said he was a victim.
Labour is unlikely to win the Commons vote on Mr Hunt, despite the Lib Dem abstention. Even if the motion was passed it would only be a symbolic blow to Mr Cameron, as he retains the final say over whether his independent adviser investigates a minister.
Shadow culture secretary Harriet Harman said: "This is an important opportunity for the House of Commons to make clear the importance it places on secretaries of state being transparent and truthful to Parliament.
"I think members from all sides of the House will want to be sure that this issue is not simply swept under the carpet.
"Misleading Parliament is not just some outdated constitutional issue - it matters."
Lib Dem MP Don Foster said Labour's decision to force a vote was a "stunt".
"It's not going to make a blind bit of difference, the Prime Minister's already made his mind up on the issue," he told BBC Radio 4's Today programme.
"The best way Nick Clegg can make clear what he believes about this is to do what he has already done.
"He doesn't endorse the position the Prime Minister's made not to refer this to the independent adviser, it's as simple as that."
Conservative Housing Minister Grant Shapps said the Lib Dems' stance was a "reminder that we have different perspectives on things".
"It's a reminder that this is two parties that have come together in the national interest to sort out these huge debt problems," he told BBC Radio 4's Today programme.
He added that the coalition was dealing with the "really big things", saying: "The important thing is that we continue to focus on the big picture, on avoiding getting into the situation of Spain or Greece."
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