The EU has been urged to remain “steadfast” over its guarantees to Northern Ireland by pro-Remain parties.
Senior figures from Sinn Fein, the SDLP, Alliance and the Greens said Brussels must remain firm in demanding a permanent “backstop” arrangement which would keep Northern Ireland aligned with the EU.
Theresa May is battling to secure a UK-wide backstop to prevent a hard border with the Irish Republic, rather than a specific measure for Northern Ireland, and also faces Tory demands to make sure that it will not become an open-ended arrangement.
But at a cross-party press conference in London yesterday, Sinn Fein vice president Michelle O’Neill said: “The backstop needs to be permanent, the backstop is the only guarantee we have in this debacle, the only insurance policy we have throughout all of the Brexit mess.
“Our message is very firm: there can be no backtracking from that position on the backstop that Theresa May and her government signed up to in December last year.”
She warned that a hard border, with infrastructure, could become a target for dissidents.
“There are still anti-peace process elements in society and any erection of hard borders on the island of Ireland will bring unwanted attention,” she said.
“While I am very cautious not to over-egg that, I think it is important that people understand that’s exactly what potentially could happen.”
SDLP leader Colum Eastwood said: “We have spent an awful lot of time trying to build the progress that we now have, the stability that we now have.
“Brexit has been like bomb going off in the middle of that stability.”
But DUP deputy leader Nigel Dodds branded Sinn Fein’s visit to Westminster to discuss Brexit as “stunt politics”.
North Belfast MP Mr Dodds highlighted Sinn Fein’s abstenionist policy, in which it refuses to take its seats at Westminster.
He said: “Sinn Fein will turn up at Westminster, do a couple of meetings, talk in front of the cameras and then disappear until they repeat the exercise in another couple of weeks. It’s a con job and a classic example of stunt politics.
“If Sinn Fein MPs were truly passionate about Brexit, they would take their seats in the Commons and actually use their votes to influence the outcome.”