Former Secretary of State Lord Hain has accused the Conservatives of "putting party before peace" by seeking an agreement with the DUP to shore up Theresa May's minority administration in Parliament.
Lord Hain, who was Secretary of State from 2005 to 2007, warned that the situation was "very damaging" at a time when sensitive talks were under way over the restoration of power-sharing at Stormont.
He said 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 local 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 that it's a very damaging situation. 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."
A Conservative spokesman said: "Peter Hain played a significant role in Northern Ireland but on this issue he is misinformed and wrong.
"An arrangement at Westminster between two parties would not in any way alter or undermine the Belfast Agreement."