(L-R) Clegg, Hunt and Cameron
Nick Clegg has exposed a coalition rift by revealing he wants David Cameron to launch an investigation into Culture Secretary Jeremy Hunt's handling of News Corp's bid to take full control of BSkyB.
The Liberal Democrat leader has repeatedly told the Prime Minister the case should be looked at by the independent adviser on ministerial interests, Sir Alex Allan.
Clegg has now asked his MPs to abstain in a Commons vote being forced by Labour on the issue.
The row comes as the Lib Dem leader gives evidence to the Leveson inquiry into media ethics.
The opposition motion calls for Sir Alex to consider whether Tory Hunt misled Parliament and failed to take responsibility for his special adviser Adam Smith.
Smith resigned after admitting his contacts with News Corporation had been too close.
The Lib Dem decision to abstain was said to have received "unanimous support" at a meeting of the parliamentary party on Tuesday night.
Lib Dem spokesman Don Foster told Sky News: "Nick Clegg is very firmly of the view that other questions, particularly in relation to the ministerial code, still remain and these should have been appropriately investigated.
"[That's] why he has not supported the Prime Minister on this occasion."
A senior Lib Dem source said Cameron was not pleased when Clegg told him the news.
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."
Clegg is reported to have discussed the matter with Cameron on a number of occasions over recent weeks, and to have told him the Culture Secretary should be subject to a probe.
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 has been breached.
He is facing accusations of double standards after calling 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 is unlikely to win the Commons vote on Hunt, despite the Lib Dem abstention - and the PM retains the final say over whether his independent adviser investigates a minister.