The Prime Minister will announce a review of devolution that will recommend improvements to inter-governmental relations, with the Union under increasing strain from the UK’s EU departure.
Mrs May will deliver her message on the eve of Boris Johnson and Jeremy Hunt making their pitch to become Prime Minister in an appearance before Scottish party activists.
The Prime Minister’s final visit to Scotland is designed to burnish her credentials as a “champion of the Union” ahead of her departure from Downing Street.
Her intervention comes amid forecasts that a no-deal Brexit would bolster support for a fresh independence referendum in Scotland and increase pressure for a ‘border poll’ in Northern Ireland.
Nicola Sturgeon claimed the visit was “a desperate act by a Prime Minister who has shown zero respect for the Scottish Parliament during her time in office”.
The First Minister added: “It’s for the Scottish people - not a Tory PM - to consider and decide what future we want for our Parliament and country.”
The two Tory leadership campaigns are understood to be frustrated by the timing of the announcement, the day before the candidates face Scottish Tory members. “It will overshadow things,” a source in the Johnson campaign said.
Mrs May has repeatedly spoken of her commitment to a Brexit deal which is good for the whole of the United Kingdom.
She has asked Lord Dunlop, a former Downing Street special adviser and Scotland Office minister, to review how Whitehall interacts with devolution in the Scottish Parliament’s 20th anniversary year.
Lord Dunlop will be asked to recommend improvements to the system in light of the UK’s imminent departure from the EU.
The peer, who will be supported by a small group of civil servants, will report back in the autumn to the winner of the battle between Mr Johnson and Mr Hunt.
One issue he is examining is whether Scotland, Wales and Northern Ireland should continue having separate Cabinet ministers representing their individual interests.
Mrs May had intended to announce the initiative at a Cabinet meeting yesterday as she works on legacy projects to mark her three years in Downing Street.
However, the meeting was postponed because the Prime Minister was forced to remain in Brussels longer than expected as EU leaders wrangled over their choice of senior posts in the European Commission. It is now expected to be held next Tuesday.
Party activists will question the two candidates at a hustings in Perth on Friday night. Ballot papers will also go out the estimated 160,000 Tory members from Friday.
Both Tory leadership candidates have agreed to calls from Scottish Tory MPs to put the Union at the heart of their Downing Street operations.
Mr Johnson has said he would take on the title of Minister for the Union, telling Tory activists in Belfast yesterday that he would personally “make sure that every policy is sense and stress tested for the benefits that it would bring to the Union”.
The former Foreign Secretary has argued that he would secure a good Brexit deal which would “strengthen our precious Union” and is the best-placed candidate to take on the SNP and Labour.
Mr Hunt, who has received the influential support of Tory Scottish leader Ruth Davidson, has described himself as a “Unionist to my finger-tips” and would “never ever” allow the UK to split up. He has pledged to create a ‘Union Unit’ in Number 10 to examine policy proposals and ensure they work for all four nations of the UK.