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 Mr 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 Mr Clegg signalled that his MPs will abstain in a vote being forced by Labour on the issue today.
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.
Tory MP Conor Burns attacked his Liberal Democrat coalition partners in an earlier Tweet:
Mr Clegg gave the news to Mr 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 Mr Clegg had discussed the matter with Mr 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 Mr Hunt's evidence to the Leveson inquiry last month - despite Mr 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.
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."
Also on the Today programme, Conservative Housing Minister Grant Shapps said the Lib Dems' stance was a "reminder that we have different perspectives on things".