THERESA May's Brexit plans suggest border checks could be set up across the Irish Sea rather than between north and south, it has been claimed.
Concerns about a 'hard border' between Northern Ireland and the Republic resurfaced yesterday following the British prime minister's speech outlining plans to leave the European single market and customs union.
However, Mrs May again said the British government does not want to see a "return to the borders of the past".
She said keeping the common travel area between the UK and the Republic would be a priority in EU divorce talks.
But she also stressed that any deal on the sensitive border issue would have to respect the "integrity" of the UK's immigration system.
"We cannot forget that, as we leave, the United Kingdom will share a land border with the EU, and maintaining that common travel area with the Republic of Ireland will be an important priority for the UK in the talks ahead," she said.
"There has been a common travel area between the UK and the Republic of Ireland for many years.
"Indeed, it was formed before either of our two countries were members of the European Union. And the family ties and bonds of affection that unite our two countries mean that there will always be a special relationship between us.
"So we will work to deliver a practical solution that allows the maintenance of the common travel area with the Republic, while protecting the integrity of the United Kingdom's immigration system.
"Nobody wants to return to the borders of the past, so we will make it a priority to deliver a practical solution as soon as we can."
Her speech has increased speculation that border checks could be maintained at ports crossing the Irish Sea rather than along the porous Northern Ireland border.
SDLP leader Colum Eastwood said if there is a new border, it "must be around the island of Ireland and not across it".
"The people of Ireland, north and south, have chosen a different future to the hard Brexit, hard border vision outlined in Theresa May's speech," he said.
"The only way Theresa May can impose new customs and immigration rules is by moving her border checks to Liverpool, Stanstead and Stranraer."
Alliance Party deputy leader Stephen Farry said the EU would have a say on any proposals for a border within its boundaries.
"The line of 'no return to hard border' is a meaningless platitude. In the event the UK ends up outside the customs union, it is impossible to avoid a physical border between it and the EU," he said.
"That will either be on the island of Ireland or down the Irish Sea. That will be up to the EU to decide as much as the UK, as it will seek to protect its own interests.
"Having either border will bring a range of significant economic, political and constitutional questions, all of which are avoided through the current arrangements and would be largely avoided through a soft Brexit with Northern Ireland as part of the customs union."
Responding to Mrs May's speech, the Irish government said its key objectives are securing trade and preserving the peace process, which included border issues.
"For Ireland, the priorities for the negotiation process that lies ahead are unchanged: our economic and trading arrangements, the Northern Ireland peace process, including border issues, the common travel area, and the future of the European Union," it said.
