The BBC reports that Gordon Brown "delivered a diatribe laced with threats" when he spoke to Nick Clegg last night by phone.
The Liberal Democrat source added that this was in contrast to "the respectful and constructive talks" between Clegg and David Cameron.
Clegg has expressed frustration with Brown's negotiating style before, most notably after the cross-party talks on MPs' expenses in June. It was Brown's obstinate approach, Lib Dems say, that turned their leader against the Prime Minister.
As a result, it's unlikely that Labour could strike a deal with Clegg without at least offering a timetable for Brown's departure. But it would be a pity if the latter's tribal style prevented us even reaching this point.
UPDATE: A Liberal Democrat source has denied that the conversation was "angry" and says it was perfectly amicable, according to the BBC's Laura Kuenssberg.
Yet, true or not, the story has already strengthened the hand of those who argue that Brown must resign as a precondition of any Lib-Lab deal.