THERESA May’s allies have moved to reassure the DUP that is she is still “a very Unionist Prime Minister”.

The Tory boss, who depends on the Northern Irish’s party’s 10 MPs to prop up her Government, has been warned that she will lose the support of her supply and confidence partners unless Northern Ireland and Britain are treated exactly the same after Brexit.

The latest row is over the backstop – in effect, what happens to the border on the island of Ireland if Britain and the EU fail to make a deal.

It has become the last major stumbling block at this stage of the Brexit negotiations.

Brussels and Dublin say if there’s no deal then the north should share their regulations, remaining in the single market and the customs union, as this is the only way to avoid a hard border.

The UK says if this happens it should only last until 2021, but the EU want no end date in any legal text.

The Europeans want it to apply “unless and until” alternative arrangements are agreed after Brexit, with the understanding that if a future UK-EU economic partnership either proves elusive or requires systematic checks on trade, the backstop could remain in force indefinitely.

That would effectively push the border between the UK and the EU into the Irish seas.

May’s former deputy Damian Green, told the BBC that the Tory leader was a “very Unionist PM, we are the Conservative and Unionist Party.

“She and the DUP are as one in wanting to make sure that Northern Ireland isn’t in some way treated differently from the rest of the United Kingdom in trade or customs terms and that is an absolutely key point in these negotiations, as the DUP have pointed out.”

The DUP’s Brexit spokesman Sammy Wilson said May had repeatedly declared herself to be a Unionist.

He warned: “She knows the consequences if she walks away from that promise, not just for Northern Ireland, because of course she will open a Pandora’s box in Scotland as well.

“I am sure the Prime Minister is well aware of the grave responsibility which lies on her shoulders.”

He added: “We are simply reminding her, you are being torn at the moment between what we believe are your gut instincts and your sound principles on the Union and at the same time the bullying, threatening behaviour of the EU.

“Go with your principles, go with your instinct, go with your responsibility to the people of the UK and show the EU negotiators the door.”

Wilson added: “We are serious about ensuring that the Unionist people of Northern Ireland are not in any way going to be affected by the narrow vision that the EU negotiators have in protecting their European project at the expense of the unity of the UK.”

Meanwhile, the Government has admitted that a no-deal Brexit could stop Eurostar in its tracks, and leave British travellers unable to use Netflix or Spotify while in EU countries.

The warnings came in the latest no-deal Brexit technical notices released by the Government, of which 29 papers were published in total.

The Scottish Government said their analysis of the documents suggested consumers could potentially face higher electricity bills as a result of changes to trading arrangements.

They also warned that extra red tape identified by the notices would lead to difficulties in recruiting key skilled workers from outside the UK – including doctors, dentists, nurses, midwives, pharmacists, vets, teachers and architects – if there is no effective replacement for current arrangements for reciprocal recognition of professional qualifications.

It could also impact Scottish seafoods, with Scottish fishermen unable to fish in EU and third country waters, or automatically unload catches in EU ports

Scottish Brexit Secretary Michael Russell said: “The reality of a disastrous no-deal Brexit looms large in this latest guidance from the UK Government.

“Potentially higher electricity prices, difficulties recruiting front line staff for the NHS and other key sectors and damaging disruption to exports will affect everyone in Scotland, but will hit our rural and coastal communities the hardest.”

Brexit Secretary Dominic Raab insisted a no deal was “unlikely”.

He said: “Securing a good deal with our EU partners remains our top priority.

“But, if the EU doesn’t match the ambition and pragmatism we’ve shown, we have the plans in place to avoid, mitigate or manage the risk of no deal and make a success of Brexit.”