Former Northern Ireland secretary Lord Hain has accused the Conservatives of "putting party before peace" by seeking an agreement with the Democratic Unionist Party to shore up Theresa May's minority administration in Parliament.
Lord Hain, who was Northern Ireland secretary from 2005 to 2007, warned that the situation is "very damaging" at a time when sensitive talks are under way over the restoration of powersharing at Stormont.
He told the Press Association the Government could not act as a "neutral facilitator" in Northern Ireland, as the Good Friday Agreement envisages, if it was dependent on one of the Northern Irish parties for its majority in the House of Commons.
"I cannot see for the life of me how you can be a neutral facilitator in bringing the parties together, at a very dangerous time for Northern Ireland politics to get self-government and the legislative assembly back up and running, when your prime ministerial life and your Government's life depends on one of the most influential parties - the biggest party - in Northern Ireland," said Lord Hain.
"I just don't see how that works. I think it's a very damaging situation."
He added: "It corrodes confidence in the negotiating process.
"I think it's very self-interested, putting party before peace in Northern Ireland - party interest before peace and stability in Northern Ireland - and I do think it's that serious."
Lord Hain said that as Northern Ireland secretary his role was crucial as a trusted channel of communication between the parties.
"We were often in a situation where the other parties, particularly in the case of Sinn Fein and DUP - who are by far the two biggest parties now, even more than they were in my time - they'd never talked to each other," he said.
"The only way they could be sure of what the other's intentions were was trusting me to be right when I told them that it was my view that, for example, if Sinn Fein did a certain thing, then Ian Paisley would agree to join the government. But they were not able to satisfy themselves face-to-face, they had to trust me that I wasn't beholden to one of them."