Welsh Secretary Peter Hain has ruled out Labour governing in coalition with Plaid Cymru if there is a hung assembly after the elections in May.
Peter Hain, Rhodri Morgan and Tony Blair claim a Tory-led coalition is planned
He said coalitions were a matter for First Minister Rhodri Morgan, but Labour would not accept one with Plaid.
In a St David's Day debate in the Commons, Mr Hain said Plaid was plotting a Conservative-led coalition.
Plaid MP Adam Price accused Mr Hain of being naive and asked whether he was "playing the outspoken maverick".
Labour was in coalition with the Liberal Democrats from 2000 to 2003 after failing to win a majority in the first assembly elections in 1999.
But Labour leaders, including Mr Morgan and Prime Minister Tony blair, used their Welsh party conference last weekend to accuse the opposition parties of planning to enter a Tory-led coalition this time round.
Plaid, however, has insisted that it would not serve under a Conservative first minister.
If Labour fails to win a majority on 3 May, Mr Hain's statement appears to mean his party might have to rely on the Lib Dems and independents to stay in power.
Asked by Plaid's Adam Price to clarify his position on a Labour-Plaid coalition, Mr Hain said: "I'm ruling it out.
"There is no prospect of that at all.
"This is a matter for Rhodri Morgan and his fellow Welsh Labour Assembly Members. But I don't think Welsh Labour would accept that."
In a statement, later Mr Price said: "Has Peter Hain discussed this with Rhodri Morgan or is he playing the outspoken maverick he has played in the past?
"He quite regularly takes a different view to those who actually make the decisions.
"Mr Hain is also naive to think that it is up to him to decide on matters to do with the Assembly. Let our colleagues in the assembly - from both parties - decide."
A spokeswoman for Mr Morgan later repeated his argument that Labour were fighting the election to form a majority government.
On the prospect of a coalition with Plaid Cymru, she said: "I don't think there is going to be much of an appetite for it".