The Liberal Democrats would find it "very difficult" to work with Gordon Brown in the event of a hung parliament, the party's deputy leader said today.
Vincent Cable was speaking following the Prime Minister's personal apology yesterday to a pensioner he had earlier labelled "bigoted".
Mr Cable, who is also the party's Treasury spokesman, said the political fall-out that had followed Mr Brown's unguarded comments was "crowding out" important debate on the economy.
The "essential" issue of Britain's massive £163 billion deficit had been overshadowed by Mr Brown's "terrible gaffe", he suggested.
Interviewed on LBC Radio, Mr Cable said: "He committed a big gaffe and he is now being punished very severely for it.
"What worried me about that exchange was that it sort of crowded out of the election debate something which probably a lot of people find a bit boring, but is absolutely essential, which is what we do about the economy.
"We are just getting launched on a proper debate about how we deal with the deficit and the Institute of Fiscal Studies report, and our tax policies... and then Gordon commits this terrible gaffe.
"He is going to be judged by it and I think people are coming to some pretty negative conclusions."
Asked if he could work with Mr Brown, Mr Cable said: "I think we would find it very difficult but, as I say, I am a Liberal Democrat, I don't choose the leader of the Labour Party, they choose him."
Pressed further on the issue of working with Mr Brown, Mr Cable said: "I think the point Nick Clegg has been making the last few days is that if, as seems increasingly likely, Labour come third, there is absolutely no basis whatever on which he could form a Government.
"So the issue becomes academic."
Asked if that meant he would work with Tory leader David Cameron, Mr Cable said: "We are willing to work with other people in the wider national interest."
Mr Cable's comments are the first made by a senior Liberal Democrat on the issue of a hung Parliament since yesterday's dramatic course of events.
Amid a media firestorm, Mr Brown went in person to the Rochdale home of 66-year-old Gillian Duffy to say sorry after he called her a "bigoted woman".
His comments, made to an aide, were picked up by a radio microphone after the widowed Labour voter had confronted him on a range of issues, including immigration.
Mr Cable was being interviewed this morning as the three party leaders prepare for tonight's third and final live televised debate.
Mr Cable said the financial crisis which had hit Greece was "very worrying".
He said: "The whole banking crisis has moved on to sovereign debt. Greece is first in line because they had a horrendous deficit, they've got enormous public debt and it's a very weakly run country, with problems like public sector pensions, which are out of control.
"I would make sure that we in Britain aren't caught up in this contagion. For that, we have to have a very clear plan to reduce our own deficit."