Theresa May has said her Government "will never be neutral" on the future of Northern Ireland as talk of a referendum on a united Ireland grows.
The Prime Minister said she believes "fundamentally" in the strength of the United Kingdom.
Her comments in PMQs come as Sinn Fein have called for a border poll following the Brexit vote.
Danny Kinahan, UUP MP for South Antrim, pressed the Government to campaign for Northern Ireland to remain part of the UK.
He said: "May I also congratulate the Government on today triggering Article 50, I know this is a momentous action for the whole of the United Kingdom.
"While I, like herself, campaigned to stay in, we recognise that the people have spoken and we offer the Ulster Unionist Party's full support in ensuring that negotiations deliver the best for the whole of the United Kingdom and particularly for Northern Ireland.
"But could I ask the Prime Minister to confirm that in the extremely improbable event that a border poll should take place regarding the future of Northern Ireland within the United Kingdom during her premiership, that the Government will fully support any official remain campaign?
"Just as the Government has done both in regard of the EU and indeed Scotland."
Mrs May said: "You are absolutely right, today of course we do give effect to the democratic decision of the people of the United Kingdom who voted for us to leave the European Union.
"And it was a call to make the United Kingdom a country that works for everyone, not just the privileged few.
"We are of course, within that, fully committed to ensuring that the unique interests of Northern Ireland are protected and advanced as we establish our negotiating position, and our position has always been clear that we strongly support the Belfast Agreement, including the principle of consent that Northern Ireland's constitutional position is a matter for the people of Northern Ireland to determine.
"But as our manifesto made clear, we have a preference that Northern Ireland should remain part of the United Kingdom, and we will never be neutral in expressing our support for that.
"And that's because I believe fundamentally in the strength of our union."