BRITAIN is heading for a hard Brexit, Theresa May will make clear today, saying the country will "leave the EU and embrace the world" in what she will describe as a great moment of national change.

In a move that will infuriate Nicola Sturgeon and set Whitehall on a collision course with Holyrood, the Prime Minister will insist the UK’s departure from the Brussels bloc will not be “half-in, half out” but complete; pointing to the country leaving the single market and the customs union.

Mrs May will say that post-Brexit the UK Government will seek a new and equal partnership between an independent, self-governing, Global Britain and its friends and allies in the EU.

Read more: Iain Macwhirter - Scraps on table after madness of an EU exit

“Not partial membership of the European Union, associate membership of the European Union or anything that leaves us half-in, half-out," she will say.

In her most important speech as premier, she will declare: “We do not seek to adopt a model already enjoyed by other countries. We do not seek to hold on to bits of membership as we leave. The United Kingdom is leaving the European Union. My job is to get the right deal for Britain as we do."

Addressing senior UK diplomats and foreign media at Lancaster House today, the Prime Minister will say: “A little over six months ago the British people voted for change. They voted to shape a brighter future for our country. They voted to leave the European Union and embrace the world.

“And they did so with their eyes open; accepting that the road ahead will be uncertain at time but believing that it leads towards a brighter future for their children and their grandchildren too. And it is the job of this Government to deliver it.”

She will explain that process will mean more than negotiating Britain’s new relationship with the EU, it will mean “taking the opportunity of this great moment of national change to step back and ask ourselves: what kind of country we want to be?”

The Prime Minister, who will attend the World Economic Forum in Davos, Switzerland later this week, will say she is determined the UK will emerge from this historic period of change “stronger, fairer, more united and more outward-looking than ever before”.

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Britain, she will argue, has the chance to be a magnet for international talent and a home to the innovators who will shape the world ahead.

“I want us to be a truly Global Britain; the best friend and neighbour to our European partners but a country that reaches beyond the borders of Europe too. A country that gets out into the world to build relationships with old friends and new allies alike.

“I want Britain to be what we have the potential and ambition to be: a great, global trading nation that is respected around the world and strong, confident and united at home.”

Speaking ahead of the landmark speech, Michael Russell, the Scottish Government Minister for UK Negotiations on Scotland’s Place in Europe, said that the threat of a Tory hard Brexit was now “getting bigger by the day”.

Warning of its disastrous economic consequences, he said: “The Tories now seem to think they can do what they want and Scotland will simply accept it; if they get away with dragging us out of the EU and the single market, then they will think they can get away with anything.”

The Prime Minister and her colleagues will be braced for not only a robust political response to her remarks but also market reaction and the prospect of a further plunge in the value of the pound.

Read more: Iain Macwhirter - Scraps on table after madness of an EU exit

Already, sterling has fallen about 20 per cent against the dollar since the EU referendum to lows last regularly seen in 1985. Yesterday, the pound hit its lowest level for three months on reports about Britain leaving the single market.

Sterling fell more than one per cent to below $1.20 before recovering slightly and it also dropped to a two-month low against the euro, falling to 1.13 euros before edging up again.

Bank of England governor Mark Carney has said UK economic growth will take a hit in the coming years as the Brexit-hit pound begins to weigh on wages and consumer spending.

Speaking at the London School of Economics, the Canadian said: “At present, households appear to be entirely looking through Brexit-related uncertainties.”

“Over the autumn, demand growth remained more resilient than had been expected, particularly consumer spending and to a lesser extent the housing market.

“That contrasted with the less sanguine assessment of financial markets as illustrated by the further, sharp decline in the sterling exchange rate over the autumn.

Read more: Herald View - The grim reality on exit from the EU

“Ultimately, the tension between consumer strength on the one hand and the more pessimistic expectations of the markets on the other will be resolved."

Mrs May’s speech will come just 24 hours or so after Donald Trump, the US President-elect, said Brexit would be “a great thing” for Britain and pledged that America would do a swift trade deal once it had left the EU.

It also comes after No 10 made clear the UK "stands ready" to become a low tax and light touch regulation economy in competition with the rest of the EU if it were unable to agree a Brexit deal with the Brussels bloc.

Tim Farron, the Liberal Democrat leader, responding to the Prime Minister's words, said: “You can call this Brexit clean, red, white and blue or whatever you want but this doesn’t disguise the fact that it will be a destructive, hard Brexit and the consequences will be felt by millions of people through higher prices, greater instability and rising fuel costs.”

In remarks partly directed at a European audience, Mrs May will insist the Brexit vote was not a rejection of shared values nor a move to become distant from Britain’s continental neighbours.

“We will continue to be reliable partners, willing allies and close friends. We want to buy your goods, sell you ours, trade with you as freely as possible, and work with one another to make sure we are all safer, more secure and more prosperous through continued friendship,” the PM will explain.

She will set out four key principles driving 12 negotiating priorities which are: certainty and clarity; a stronger Britain; a fairer Britain and a truly global Britain.

It is expected she will outline what the 12 negotiating priorities are in another unreleased section of her speech.

“We have 12 objectives that amount to one big goal: a new, positive and constructive partnership between Britain and the European Union,” she will add.

Meantime, it is expected that the ruling by the UK Supreme Court on whether or not MPs should have a vote on Mrs May’s Brexit strategy before Article 50 is triggered by the end of March will be announced next week.