FORMER first minister Lord David Trimble has said a post-Brexit border on the Irish Sea would be "dangerous" for unionists.
A sea border would mean Northern Ireland remaining within the single market and customs unions while Britain leaves.
Taoiseach Leo Varadkar has said there was no proposal that the Irish Sea should become the new frontier with the UK after Brexit.
However, Lord Trimble, former leader of the Ulster Unionists, told the Spectator magazine a sea border would endanger the Good Friday Agreement.
"The Belfast Agreement recognises British sovereignty in Northern Ireland, and recognises Northern Ireland as part of the UK," he said.
"To have provisions treating us as if we are not part of the UK is clearly contrary to that agreement and is something no unionist is going to support.
"Once it begins to dawn on the unionist electorate that the Irish government is trying to break up the UK then we are into very dangerous territory indeed.
"The Government needs to quash this idea very quickly, and make it clear that we will not have any damage done to our constitution."
Speaking in a round of broadcast interviews during a visit to Iraq, British Prime Minister Theresa May reiterated her opposition to a hard border.
"We are maintaining the common travel area that has been in place since 1923, so long before either Ireland or the UK were members of the EU. That is part of what we have agreed with the EU," she said.
"We also want to ensure that trade between Northern Ireland and the Republic can continue, but also that trade between the Republic of Ireland and the rest of the UK can continue. I want to get on to those trade talks and I am very clear - no hard border."
Meanwhile, the head of Northern Ireland's civil service has said planning for Brexit would be helped "considerably" if Stormont ministers were in place.
The north has been without a devolved administration for almost a year.
David Sterling said it would be helpful to have ministers who could "advance Northern Ireland's cause with a single voice."
