Party’s Commons leader warns it is not bluffing when it says it will not accept Irish Sea border

The Democratic Unionist party has threatened to pull the plug on Theresa May’s government, warning it is not bluffing when it says it will not accept a border in the Irish Sea.

Nigel Dodds, the leader of the DUP in the House of Commons, said the party would vote against May if she returned from Brussels with a deal that involved new checks on goods coming into Northern Ireland from Britain.

“We will vote against it. We will vote for our red lines,” he told the Guardian.

The DUP leader, Arlene Foster, had earlier said the party’s confidence and supply arrangement with the Conservatives was “party to party” and not with May herself, while the European commission accused Westminster decision-makers of being “completely irresponsible” over Northern Ireland.

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Dodds said the DUP was not afraid to take action if the prime minister betrayed the party by going back on her promise that Northern Ireland would not be left with different trading and customs arrangements from Britain.

“In Northern Ireland we’ve grown up with a lot of fears and issues that we’ve faced, frankly being afraid of what Theresa May may do is not one of my biggest fears,” he said.

After a presentation at a fringe event at the Tory party conference in Birmingham, he made clear the party was not intending to abandon its confidence and supply agreement but that it would not prop up a prime minister that left Northern Ireland “semi-detached” from the UK.

He said the party would not be fobbed off with guarantees in a political deal between the EU and the UK that would accompany any Brexit deal.

“The deal on the future relationship will have to be crystal clear and not a fudge, we won’t settle for any vague outline of a future relationship in exchange for a backstop, that is simply not going to happen,” he said.

His remarks will dash hopes that delicate ongoing talks over a compromise on the Irish border could be hammered out before the next EU summit later this month.

Foster had earlier openly spoken about the possibility of working with Boris Johnson, in a clear signal to May that she could not take their support for granted.

Facebook Twitter Pinterest Boris Johnson speaks at the Tory party conference on Tuesday. Photograph: James McCauley/Rex/Shutterstock

“Theresa May understands very well that we are not bluffing on this issue … there is far too much at stake for us as unionists, but also economically for Northern Ireland,” she told the Guardian.

Foster and Dodds are due to meet May in a side meeting in Birmingham on Tuesday afternoon and will be repeating their message to her, they said.

The Scottish National party leader, Nicola Sturgeon, said the threat to the confidence and supply agreement was a major development for May. “I suspect this is a far more significant development today than the latest Boris Johnson circus,” she tweeted.

Nicola Sturgeon (@NicolaSturgeon) I suspect this is a far more significant development today than the latest Boris Johnson circus. https://t.co/UuAqT4eyO5

The EU has been hoping to break the deadlock on the Irish border by arguing checks on goods could be made on ships and at ports in Britain.

Earlier the commission vice-president, Frans Timmermans, condemned the attitude towards Northern Ireland in Westminster.

“It is central to our responsibility to do as little harm as possible in these negotiations,” Timmermans said. “And to just brush it aside in London is completely irresponsible.”

The commission’s president, Jean-Claude Juncker, said: “When it comes to the border between the Republic of Ireland and Northern Ireland, we are sticking to the point of view that we have expressed so many times: Ireland first.”

The UK and the commission have agreed that there needs to be a backstop solution to avoiding a hard border. It would kick in after Brexit and the transition period ending in December 2020 should a trade deal or technological solution not be in place by that point.

May has said that no British prime minister could accept the commission’s proposal to allow Northern Ireland to maintain key benefits of EU membership by remaining in the bloc’s customs union and under single market legislation.

She said Whitehall would instead put forward a new proposal under which the whole of the UK would remain temporarily in a customs arrangement. Stormont would also be asked if it wanted to stay under EU single market rules.

Other key figures, including Johnson have claimed that the issue has been overblown.

Speaking to MEPs in Strasbourg, Timmermans said he was astonished by the failure in the UK to see the importance of EU membership to keeping the peace in Northern Ireland.

The former Dutch foreign minister told MEPs: “How is it possible that people coming from the UK maintain that there is no link between union and the Good Friday agreement. The European Union is an integral part of the success of the Good Friday agreement.

“It would not have been possible without the European Union. The Good Friday agreement was one of the most historical achievements … We are all responsible for maintaining that legacy and preventing the island of Ireland from going to a past that I knew as a young man.”