David Cameron defied repeated calls from Nick Clegg to launch a sleaze probe into Culture Secretary Jeremy Hunt's handling of the BSkyB bid, it has been revealed.
The Liberal Democrat leader believes the independent adviser on ministerial interests, Sir Alex Allan, should be brought in to consider the case.
The behind-the-scenes coalition clash emerged as Mr Clegg signalled that his MPs will abstain in a vote being forced by Labour on the issue.
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.
Mr Clegg conveyed 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 added.
"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 that 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.