Theresa May has warned her successor not to trust Nicola Sturgeon as she admitted the Union was in grave danger.

In a blunt message to Jeremy Hunt and Boris Johnson, the Prime Minister said the SNP could not be relied upon to act in “good faith”.

Warning "we do not have good will on all sides", she said the next occupant of Number 10 must actively strengthen the Union or it would be "too late".

She said: “Over the last three years I have learned that while other parties can be relied on to work with the UK Government in good faith to make devolution a success, an SNP Scottish Government will only ever seek to further the agenda of separation.

“That, I am afraid, is simply a fact of political life in the UK at the moment.”

"That fact puts an additional responsibility on the UK Government. If we do not do all we can to realise the full benefits of the Union – no one else will.

"If we do not use every policy lever within our reach to strengthen that Union – no one else will.

"And if we do not make realising the full benefits of being a United Kingdom of four proud nations and one united people our priority now, then in the future it may be too late."

In her last official speech north of the border before her successor is named on July 23, Mrs May warned the next Prime Minister not to take the Union for granted, but actively promote and protect it from Nationalists working to bring about its end.

She agreed with former Labour PM Gordon Brown that the UK was under threat.

Describing Brexit as “a profound constitutional change putting political and administrative strains on the Union”, she said: “When Gordon Brown recently said that he fears the Union is ‘more imperilled now than it has ever been’ he voiced the fears of many.

“I care passionately about our Union. I certainly do not underestimate the scale of the challenge it faces. But I am optimistic about its future.”

Mrs May was speaking to party activists in Stirling about strengthening the Union after 20 years of devolution, announcing a review of how UK Government departments could do so led by former Scotland Office minister Lord Andrew Dunlop.

She said: “Today, the only threat to devolution comes from those parties who want to end it by breaking up the United Kingdom.

“For those of us who believe in the Union, devolution is the accepted and permanent constitutional expression of the unique multinational character of our Union.

“It was ironic that the UK Government’s sincere efforts to ensure that Brexit had no unintended consequences for the UK’s internal market was dismissed as a ‘power-grab’ by the SNP. 

“A UK Government which had enthusiastically launched and implemented the Smith and Silk Commissions – transferring sweeping powers over tax and welfare - stood accused of using Brexit as the cloak behind which to claw-back powers over food labelling and fertiliser regulations. On one level the allegation is simply absurd.

“On another, it highlights a challenge which faces the UK Government as it seeks to act in the best interests of the whole UK.  Whereas the UK Government is invested in the success of devolution, it would suit the political aspirations of the present Scottish Government for devolution to fail, or to be seen to fail.”

She repeatedly cited the pledge by Alex Salmond and Nicola Sturgeon that the 2014 independence referendum would be a once in a generation or once in a lifetime decision.

She said: “If a majority had supported independence, the UK Government would have accepted that result – no question. But the people of Scotland did not vote for independence.

“A clear and decisive majority of ten percentage points gave the Union their backing and their decision should be respected. 

“So when Nicola Sturgeon requested of the UK Government in 2017 the power to legislate for a second independence referendum, just three years after that historic vote, I had no hesitation in firmly saying ‘no’. In the future, it will be for others to decide based on the prevailing circumstances how to respond to separatism. But the principle is clear – the Union can and will only prosper if it enjoys the support of its people.”

The speech was delivered on the eve of Mr Johnson and Mr Hunt appearing at the sole Scottish hustings in the Tory leadership contest in Perth tomorrow. 

Mr Johnson has promised to deliver Brexit by Hallowe’en “do or die”, even if it means a no-deal that could boost support for the SNP and independence.

Mr Hunt has said he believes in the Union “with every fibre of my being”, but has also set a no-deal deadline of September 30, saying he would be ready to take the UK out of Europe on WTO terms if there was no realistic chance by then of a renegotiated deal. 

Ahead of Mrs May speech, Mr Sturgeon predicted the Prime Minister’s ultimate legacy would be Scottish independence.