British Prime Minister Theresa May has stirred the ire of Scottish nationalists with a speech that would focus on the "duty" of the leader of the United Kingdom government to strengthen the union of England, Scotland, Wales and Northern Ireland.

The prime minister was in Scotland to speak to Conservative Party activists on Thursday, a day before the first hustings north of the border between the two party leadership candidates Boris Johnson and Jeremy Hunt, in Perth.

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.

Scotland's First Minister 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".

Sturgeon said neither Johnson nor Hunt would be able to "undo the damage" to the union inflicted by May's handling of Brexit.

In her speech, May would describe preserving the union as "one of the first and greatest" duties of whoever leads the government, calling for politicians to come up with new ways of ensuring its longevity.

"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.

"One of the first and greatest is the duty you owe to strengthen the union. To govern on behalf of the whole United Kingdom.

"And to ensure that we can go on facing the future together, overcoming obstacles together, and achieving more together than we ever could apart – a union of nations and people."

May has repeatedly spoken of her commitment to a Brexit deal which is good for the whole of the UK, and has ordered a review of UK government departments to ensure they work in the best interests of devolution, which will suggest where improvements might be made.

One issue to be examined is whether Scotland, Wales and Northern Ireland should continue having separate Cabinet ministers representing their individual interests.

Ahead of the prime minister's arrival in Scotland, Sturgeon claimed that the country was "heading inexorably towards independence" and that the Conservatives were "running scared of the rising tide of support for independence".

"It's for the Scottish people – not a Tory PM – to consider and decide what future we want for our Parliament and country."

She added: "The Tories' behavior 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.

"People across Scotland can now see that more plainly than ever. Theresa May's so-called review of devolution is too little, too late".

Both Tory leadership candidates have agreed to calls from Scottish Tory MPs to put the union at the heart of their Downing Street operations.

Ballot papers will go out to the estimated 160,000 Tory members on Friday. The winner of the leadership contest is expected to be announced on July 23.