A "more united" Britain is needed for the country to get the best from Brexit, Theresa May will say today.
The prime minister will tell Tory activists that the break from Brussels will require "one United Kingdom, all pulling together to get the best outcome".
In a speech at the Conservative Spring Forum in Cardiff, Mrs May will say that she will "always fight to strengthen and sustain this precious, precious union".
It comes just a day after she rejected Nicola Sturgeon's demand for a second referendum on Scottish independence before Brexit is finalised.
Mrs May is expected to trigger Article 50 and formally start the process of withdrawing from the European Union by the end of the month.
"The coming negotiations with the EU will be vital for everyone in the United Kingdom," the prime minister will say.
"Every person, every family, every business, every community the length and breadth of the United Kingdom - England, Scotland, Wales and Northern Ireland."
It is "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."
Ms Sturgeon wants a second vote on pulling Scotland out of the UK as soon as next autumn, with the Scottish parliament set to vote on the matter next Wednesday.
Ms Sturgeon responded by saying Mrs May's announcement could make it "the day the fate of the union was sealed".
As she sets out her "Plan for Britain", the prime minister will be hoping for more positive headlines after a bruising week which has also seen 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.