The union has a bright future if "those of us who care for it act wisely", Theresa May has said on her last visit to Scotland as Prime Minister.

Speaking 24 hours before the contenders to replace her, Boris Johnson and Jeremy Hunt, arrive in Perth for Conservative leadership hustings, May acknowledged Brexit has put "political and administrative strains" on the union.

Her veiled remarks follow her deputy David Lidington's warning that no-deal Brexit would put "much greater strain" on the UK's ability to stay together.

In a speech in Stirling, the Prime Minister said: "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.

"The Union has proved a remarkably durable and flexible relationship over the centuries, evolving to meet the needs and aspirations of the people of these islands.

"I believe if those of us who care for it act wisely, if we draw on its great strengths and think creatively about how to build on them in the years ahead, its future can and will be a bright and prosperous one."

It comes a day after she announced a review of Scottish devolution in a bid to ensure the current constitutional set-up is "working as best as it can".

In her speech on Thursday, May claimed both candidates for Number 10 "are supportive of the review", which will be led by former Scotland Office minister Lord Dunlop.

The Prime Minister refused First Minister Nicola Sturgeon's formal request for an order to hold a second independence referendum in 2017.

On how her successor should approach a similar request in future, May said: "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."

In a brief Q&A session with journalists after her speech, May was pressed by STV's political editor Colin Mackay on if the union had been weakened on her watch - and when she expects to see a fresh independence vote.

The PM replied: "In 2014, the then-first minister and his then-deputy, who is of course now first minister, said that that referendum was a once-in-a-generation or a once-in-a-lifetime event.

"I think it's time that the SNP government here in Scotland paid attention to the day job and stopped obsessing about independence."

Speaking to STV News earlier, the First Minister accused the outgoing Prime Minister of boosting independence support by treating Scotland in a "high-handed, arrogant way".

On how May's successor should deal with calls for indyref2, Sturgeon said: "It's perfectly legitimate to oppose independence, but it's not for Jeremy Hunt or Boris Johnson or even for me to decide Scotland's future.

"It's for the people of Scotland to decide the future of our country."

She added: "Listening to them, I don't see either of them showing any sense of wanting to listen to Scotland, or understand Scotland's different views or accommodate Scotland's different views.

"It seems for the Tories these days, strengthening the union doesn't actually involve paying any attention or giving any respect to Scotland's views."