THERESA May will today provoke a row with Nicola Sturgeon by insisting it is her "duty to represent all of our United Kingdom” in the Brexit talks as she pledges to strike a deal with the EU that is broader and deeper than any free trade agreement anywhere in the world.

With the political rhetoric intensifying over Britain’s withdrawal, the Prime Minister will insist in a keynote speech that she is seeking to achieve an “enduring solution,” that forges a new relationship with the EU that “goes beyond the transactional to one where we support each other’s interests”.

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Addressing an audience at the Mansion House in the City of London - switched from north-east England because of the adverse weather conditions - she will set herself five tests, including a desire to “strengthen our union of nations and our union of people”.

Mrs May will say: “We must bring our country back together, taking into account the views of everyone who cares about this issue, from both sides of the debate. As Prime Minister it is my duty to represent all of our United Kingdom, England, Scotland, Wales and Northern Ireland; north and south, from coastal towns and rural villages to our great cities.”

Her remarks are likely to enrage the First Minister and her SNP colleagues, who claim there has not been proper engagement between Whitehall and Edinburgh on Brexit. This week, Ian Blackford, the Nationalist leader at Westminster, accused UK ministers of adopting a “Big Brother” approach and of treating the devolved administrations “abominably”.

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The country faces a constitutional crisis with the Scottish Government rejecting Whitehall’s latest offer to break an impasse over the UK Government’s flagship Brexit Bill, which Edinburgh still regards as a “power-grab”.

With the possibility of no agreement on the legislation, the Scottish Government has announced that its own Brexit Continuity Bill will be fast-tracked through Holyrood; a final vote is due within three weeks.

“This is not one of these situations where we have both got our positions and we meet in the middle in some vague way. This is a fundamental issue of principle,” declared Ms Sturgeon.

“The latest proposals from the UK Government would involve consultation with the Scottish Parliament and other devolved administrations around issues that are part of our responsibility.

“I don’t think consultation is enough; it should be the consent of the Scottish Parliament that is required,” added the FM.

Next week, UK and Scottish ministers are due to meet one again at the Joint Ministerial Committee to try to end the political deadlock on the EU Withdrawal Bill with a May-Sturgeon meeting expected before March 22; thought to be the last date the UK Government can table an amendment to its bill.

On Thursday, following a special Cabinet meeting to discuss her keynote speech, Mrs May met Donald Tusk over a working lunch of lemon sole in Downing Street with the Irish border issue front and centre. No 10 said the meeting had been “positive and constructive”.

It is believed the European Commission President challenged the PM to come up with "a better idea" than the EU's controversial plan to keep Northern Ireland in the customs union if she wanted to prevent a hard border.

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Ahead of the bilateral, Mr Tusk poured cold water on Britain’s desire for “as frictionless trade as possible,” making clear there could be "no frictionless trade" with the EU if the UK were outside the customs union and single market.

In Brussels, Michel Barnier, the EU’s chief negotiator, pledged “no compromise” on the European Union’s customs rules

“We have learnt the UK wants to leave the single market and the customs union and our common supervision and enforcement structures. So the UK is closing the door on itself,” he declared.

Mr Barnier stressed the EU27 would be willing to look at any “better ideas” the UK might have to avoid a hard border between Northern Ireland and the Republic but noted how Britain’s decision to leave the customs union had “consequences”. “Being outside always involves customs procedures and checks,” he stressed.

Piling more pressure on the UK, Irish senator Neale Richmond, the European affairs spokesman for the Fine Gael party that leads the Dublin Government, claimed London had provided "zero detail" on its alternative technological approach to avoiding a hard border while Phil Hogan, Ireland’s EU Commissioner, condemned what he called the “hysterics” in response to Brussels legal draft on the withdrawal agreement.

“If you start off in a negotiation with a lot of red lines – no customs union, no single market, no European court of justice – well then you run out of options and this is the problem with the UK Government,” he added.