Mrs May will urge her successor to make safeguarding the union their “greatest duty” and also defend her own record on strengthening the UK during her three years in Downing Street.

Leadership candidates Boris Johnson and Jeremy Hunt will address a hustings event in Perth tomorrow night, days before postal ballots are sent to Tory party members who will elect Mrs May’s successor.

The Tory leader will set out plans for a review of the UK government’s approach to devolution as she addresses an audience in Scotland this afternoon, but it has been dismissed as “too little, too late” by the SNP leader.

Theresa May leaves Downing Street to go to Prime Ministers Questions yesterday. Picture: PA

The First Minister also warned that Mrs May’s successor will never “undo the damage” she has done to the Union.

The SNP government at Holyrood is seeking to stage a second referendum on leaving the UK next year, in response to Brexit uncertainty. Recent polls indicate that support independence is now neck and neck with staying in the UK among Scots.

A YouGov poll published a fortnight ago indicated that support for independence now stands at 49 per cent, with backing for the Union on 51 per cent. However, if Mr Johnson wins the Tory leadership race, as expected, and becomes Prime Minister, 53 per cent of Scots say they would vote to leave the UK.

“Scotland is heading inexorably towards independence – that will be Theresa May’s legacy,” Ms Sturgeon said ahead of the Prime Minister’s visit.

“The Tories’ behaviour towards Scotland in the three years since the Brexit vote has been high-handed, arrogant and dismissive.

“They have demolished any notion of a respect agenda and have destroyed their own claims that the Union is in any meaningful way a partnership of equals.”

With both candidates agreeing to calls from Scottish Tory MPs for the UK government to work harder on defending the Union, Mrs May will urge future policy makers to think creatively to ensure “we can go on facing the future together”.

“I am confident that whoever succeeds me in 10 Downing Street will make the Union their priority,” she will say.

“He will be building on work done over the last three years, during which time strengthening the Union has become an explicit priority of government.

“The job of Prime Minister of the United Kingdom of Great Britain and Northern Ireland brings with it privileges and responsibilities which you only really feel once the black door closes behind you.”

Mr Johnson has pledged to take on the role of Minister for the Union to ensure that all policies are “sense and stress tested” for all parts of the UK, while Mr Hunt has said he would created a ‘Union Unit’ at the heart of his Downing Street operation.

Former Scotland Office Minister Lord Dunlop is to lead a review of the devolution settlement at Westminster to ensure that all UK departments take account of the devolved nations when making policy.

But Ms Sturgeon said: “Theresa May’s so-called review of devolution is too little, too late – it is reminiscent of John Major’s doomed ‘taking stock’ exercise in the 1990s, which only accelerated the pace towards the devolution referendum and the creation of Holyrood.”

Scottish Tory MPs were only told of plans to hold a review two weeks ago, when they held a meeting with the Prime Minister and her chief of staff Gavin Barwell. “She said all the right things, but the point was made that it is ridiculous that we are only having this conversation now, two years on from the general election and with a prime minister that is on her way out,” one MP told The Scotsman.

Scotland Office minister Lord Duncan said the review was a “simple, straightforward way of making sure devolution is working as best as it can be”.

Tory leadership hopefuls promise to defend the Union as polls show a 50/50 divide