Rishi Sunak promised to be "resolute" in defending Northern Ireland as he seeks a deal to rewrite the terms of its post-Brexit arrangements.
The Prime Minister indicated that he would put any new deal to a vote in the Commons, risking a showdown with Eurosceptics on the Tory benches.
The Government and the European Union are still negotiating changes to the Northern Ireland Protocol, the arrangements aimed at preventing a hard border with Ireland.
Mr Sunak told MPs: "I am a Conservative, a Brexiter and a unionist, and any agreement that we reach needs to tick all three boxes.
"It needs to ensure sovereignty for Northern Ireland, it needs to safeguard Northern Ireland's place in our union, and it needs to find practical solutions to the problems faced by people and businesses.
"I will be resolute in fighting for what is best for Northern Ireland and the United Kingdom."
The Prime Minister spoke to European Commission president Ursula von der Leyen on Tuesday night, Downing Street said.
