Northern Irish protestant politicians who are propping up British Prime Minister Theresa May's minority government reacted with fury on Friday to a report that Ireland wants the Irish Sea to be its border with Britain after Brexit.
They described the idea as "absurd" and "unconstitutional". One senior member of the Democratic Unionist Party (DUP) suggested it now meant "a very hard border" returning to the island.
It is a particularly sensitive issue given the decades of violence in the province that ended in a peace deal less than 20 years ago.
The border between the Irish Republic, a member of the European Union, and the British province of Northern Ireland will be the only land frontier between the United Kingdom and the European Union once Britain leaves the bloc in early 2019. Politicians in London, Dublin, Belfast and Brussels have all said they want to avoid the return of a "hard border" on the divided island, although no progress has yet been made. It is one of the key decisions to be made before a Brexit agreement can be reached.
In Friday’s Times: Move border to Irish Sea after Brexit, says Coveney pic.twitter.com/d4OT3kyhS8— The Times IE (@thetimesIE) July 27, 2017
The only way for the DUP to avoid a hard border with Ireland and a border in the Irish Sea is to stay in the customs union. End of. — Will Straw (@wdjstraw) July 28, 2017
In the latest wrinkle, The Times newspaper reported that the Irish government's preferred option is for customs and immigration checks to be located away from the land border and at ports and airports.
That would effectively draw a new border in the Irish sea, possibly meaning one between Britain and its province.
Ian Paisley Jr, one of the 10 DUP members of the British parliament allowing May's government to stay in power, went on Twitter to condemn the idea.
"1 of 2 things will now happen 1. A very hard border 2. Ireland will wise up and leave the EU," he said.
Reports Dublin is pushing for Irish Sea to become post-Brexit border with UK. Trouble ahead for Theresa May? More @GMB— Juliet Dunlop (@julietdunlop) July 28, 2017
May has said she wants the post-Brexit border to be as seamless as possible. Dublin, meanwhile, fears anything resembling the land border posts of the past could pose a risk to peace.
About 30,000 people cross the current, invisible frontier each day for work and many farms straddle both sides of the border.
Any hindrance to cross-border trade would hit Northern Ireland harder. The Republic accounts for 25% of Northern Irish exports outside the UK, compared with just 1.4% going the other direction.