The Prime Minister will appeal to the Scottish people to get fully behind the exit negotiations, saying: “We are one people.”
And she will make the audacious claim that the Brexit vote was “an instruction to change the way our country works – and the people for whom it works – forever”.
Speaking in Cardiff, Ms May 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 is essential that we get the right deal, and that all of our efforts and energies as a country are focused on that outcome.
“We need to do so united, as one United Kingdom, all pulling together to get the best outcome. 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.”
The speech will come the day after Ms May refused the SNP’s demand for a pre-Brexit referendum on Scottish independence – to Nicola Sturgeon’s fury.
Unveiling her “Plan for Britain”, the Prime Minister will promise to “seize the moment of opportunity to shape a brighter future for the whole country”.
She will say: “Our Plan for Britain is a plan for a brighter future. A plan to make the most of the opportunities ahead and to build a stronger, fairer Britain that is more united and more outward-looking.
“A plan to get the right deal for Britain abroad yes, but also a better deal for ordinary, working people here at home.”
At the Conservative Spring Forum, Ms May will argue her “ambitious economic and social reforms” have three key strands, to build:
* A stronger economy – tackling the problem of low productivity and securing “the high-paid, high-skilled” jobs of the future.
* A fairer society – breaking down “the barriers of privilege” and spreading opportunity and prosperity around the country.
* A more united nation – to “strengthen the bonds of our precious Union”.
The wide-ranging plan – a manifesto in all but name – may be seen as a further indication that the Prime Minister is considering an early general election, despite her frequent denials.
This week’s U-turn over higher national insurance contributions proved the danger of relying on a small Commons majority, some Tories argue – with much bigger battles over Brexit to come.