Theresa May will pledge to forge a "more united" nation as she begins the Brexit process.
In a speech at the Conservative Spring Forum in Cardiff, the Prime Minister will stress that the break from Brussels will require "one United Kingdom, all pulling together to get the best outcome".
The speech on Friday to Tory activists, which will see Mrs May stress the importance of the "precious" union, comes after the Prime Minister rejected Nicola Sturgeon's demand for a referendum on Scottish independence before Brexit is finalised.
Mrs May, who is expected to trigger Article 50 and formally start the process of withdrawing from the European Union by the end of the month, will say: "The coming negotiations with the EU will be vital for everyone in the United Kingdom.
"Every person, every family, every business, every community the length and breadth of the United Kingdom - England, Scotland, Wales and Northern Ireland."
It was "essential that we get the right deal" and that requires working together, she will say.
"That is what we have always done when faced with challenges.
"We have pulled together as one and succeeded together.
"We are four nations, but at heart we are one people.
"And I will always ensure the voices and interests of England, Scotland, Wales and Northern Ireland are represented as we negotiate to leave the EU.
"And I will always fight to strengthen and sustain this precious, precious union."
She will insist that the EU referendum result was not just a demand for Brexit, but an instruction to change the way the UK works forever.
Mrs May will say: "We will get the right deal for Britain abroad - forging a new partnership with our friends and allies in Europe, but looking beyond Europe to build relationships with old friends and new allies around the world too.
"And at the same time, we will pursue the ambitious economic and social reforms we need to make Britain work for working people."
Her goal is "building a stronger, fairer Britain that our children and grandchildren will be proud to call home".
Mrs May will set out her "Plan for Britain" at the end of a bruising week which has seen Ms Sturgeon call for a second independence referendum, Chancellor Philip Hammond forced into a humiliating U-turn over the Budget's National Insurance rise and the Tories fined a record £70,000 for breaching election spending rules.
She will say the plan puts "strengthening and sustaining" the union at its heart.
"It means taking the big decisions when they're the right ones for Britain in the long-term.
"Putting the national interest above any other consideration.
"And it means ensuring that we act in the interests of the whole country - creating jobs and supporting cities, towns and communities right across our United Kingdom."
Mrs May will hope the Plan for Britain is more successful than the Government website for it - which crashed after going live on Thursday.