DUP threaten to torpedo Theresa May's Brexit proposals and say they could vote with Labour

The PM's allies said they would not tolerate a border of "any kind" down the Irish Sea

Theresa May's DUP allies have threatened to torpedo her Brexit plans and vowed to vote with Labour if the proposals breach their red lines.

Westminster leader Nigel Dodds said his party would not tolerate a border of "any kind" down the Irish Sea, effectively ruling out potential regulatory checks at Irish Sea ports as part of a backstop agreement.

Mr Dodds also indicated his party would walk through the voting lobbies with Jeremy Corbyn, despite their animosity towards the Labour leader over his perceived support for republicanism.

His remarks will come as a blow to the prime minister who needs the support of the Northern Irish party for any Brexit deal she secures with Brussels.

Mr Dodds told a fringe event at Conservative conference: "Let us be clear that for the DUP there will be no border of any kind down the Irish Sea - customs, regulatory, political, constitutional or otherwise because we are part of the UK and we will leave the EU together and as one nation.

"The danger of this Irish backstop is that it has the potential, to not only separate Northern Ireland from the rest of the UK, in any divergence that there may be from it, it also has the potential to shackle the UK for generations to come in its relationship with Europe.

"That is why it is critical that we get it right and the DUP will not be signing up to any backstop unless we are sure that every line of it complies with our requirements that I have set out."

The Belfast North MP expressed concerns over "a resurgent Labour Party, which is fizzing with ideas and attracting a lot of support".

Speaking to reporters after the event, Mr Dodds said his party would vote against any Brexit deal Ms May brought to parliament if it did not meet their requirements.

Asked if he would vote with Mr Corbyn, he said: "We have our red lines, we will make our judgement on the basis of what’s in the interests of Northern Ireland and the UK.”

Asked again, Mr Dodds said: “We will make the decision on how to vote on any proposition brought forward by the prime minister in respect of whether it meets our red line which is the integrity of the United Kingdom.

"If others join with us on that, great.”

At the same meeting, DUP leader Arlene Foster fired off a stern warning to Theresa May that her party could not accept a backstop arrangement that threatened the integrity of the United Kingdom.

She said: "We have to ensure that there is no border down the Irish Sea. We have to continue to have the integrity of our country maintained from a constitutional point of view and also from an economic point of view.

"We do absolutely want to see a deal that delivers for Northern Ireland and for the whole of the United Kingdom. But we are not going to allow the UK to be broken up by Brussels or by anybody else."

She added: "[EU chief negotiator Michel] Barnier cannot annexe off part of the United Kingdom so he can say that that’s the way forward.

"We are not bluffing on this issue."

The event was attended by a string of cabinet ministers - in a sign of the DUP's strategic importance - including defence secretary Gavin Williamson, Northern Ireland secretary Karen Bradley and the chief whip Julian Smith.

It comes after Ms Foster said the Good Friday Agreement is not "sacrosanct", insisting the landmark peace treaty could be altered to accommodate a Brexit deal.

In an interview ahead of the event, she said the historic agreement "could evolve" in the EU context and expressed frustration at warnings from pro-EU campaigners over Brexit's potential to undermine aspects of the international treaty.