The DUP MP Sammy Wilson has rejected suggestions that Northern Ireland’s place within the UK will be jeopardised by the new Brexit arrangements.
The DUP is unhappy about aspects of last week’s trade deal and will vote against it at Westminster this week.
But the party still believes Brexit is the best way forward.
Mr Wilson, the DUP’s Brexit spokesman, said he believed the economy would grow as a result of the UK’s withdrawal from the EU, strengthening the union.
The post-Brexit trade deal has been approved by EU ambassadors, paving the way for it to take effect on 1 January.
MPs will return to the House of Commons on Wednesday to vote on the deal.
The DUP, the Alliance Party and the SDLP have all indicated they will vote against it.
But Mr Wilson said: “I can guarantee that once people in Northern Ireland – unionists and nationalists, Catholics and Protestants – look at the benefits of being part of the fifth biggest economy in the world, and an economy that I believe will grow as a result of Brexit and strengthen as a result of Brexit… I believe that they will understand that their future lies with Britain.”
The East Antrim MP said being within the UK was much better than any alternative arrangements.
He said the Republic of Ireland “gets tossed in the sea of economic storms, worldwide economic storms, like a cork”.
He added: “I know that nationalists, or some nationalists, think that Brexit will be the catalyst for a united Ireland.
“The only catalyst for a united Ireland will be when the people of Northern Ireland come to the conclusion that it is not worthwhile being part of the UK.”
The DUP’s decision to vote against the post-Brexit trade deal was confirmed on Monday evening.
A party spokesperson said: “Whilst we accept that this agreement does bring about zero tariff and quota arrangements between the UK and the European Union, thus removing many goods from attracting tariffs between Great Britain and Northern Ireland, the fact remains that this agreement does not assist Northern Ireland in the context of having to operate under the Northern Ireland protocol.”