The Democratic Unionist Party has said it “will not accept” a deal on the Irish border being brokered by Theresa May, threatening her with a fresh crisis.

Staging an emergency press conference in Belfast, Arlene Foster, the DUP leader, ruled out accepting any move “which separates Northern Ireland economically or politically from the rest of the United Kingdom”.

The warning comes as a deal appeared close in Brussels under which the UK would allow Northern Ireland to maintain “regulatory alignment” with the EU.

Ms Foster did not directly repeat the DUP’s threat last week to pull the plug on the deal that props up the Prime Minister in No 10.

Instead, she described the Irish Government’s suggestions that the UK was poised to bow to its concerns over the post-Brexit border “speculation”.

But she added: “We have been very clear. Northern Ireland must leave the EU on the same terms as the rest of the United Kingdom.

“We will not accept any form of regulatory divergence which separates Northern Ireland economically or politically from the rest of the United Kingdom.

“The economic and constitutional integrity of the United Kingdom will not be compromised in any way.”

Ms Foster said No 10 should be in no doubt about its stance, adding: “Her Majesty’s Government understands the DUP position.“

Ms May did not comment on the reports as she arrived for a key working lunch with Jean-Claude Juncker, which will be followed by talks with European Council president Donald Tusk.

Preventing the return of a hard land border in Ireland has emerged as the key hurdle to getting agreement from the EU to move the Brexit negotiations on to future trade.

Regulatory alignment could see both Ireland and Northern Ireland following the same rules governing trade, to ensure that goods can continue to move freely across a “soft” border with no checks.

But a deal could put the Prime Minister’s domestic position in peril, because she depends on the votes of the 10 DUP MPs in order to pass a Budget and Brexit legislation.

Earlier, Ms May's official spokesman insisted any agreement on the Irish border would protect the "territorial and economic integrity" of the UK.

Ahead of the meetings in Brussels, he told a regular Westminster briefing: "The PM has been clear that the UK is leaving the European Union as a whole and the territorial and economic integrity of the United Kingdom will be protected."