The Foreign Secretary, Jeremy Hunt, has refused to rule out a second independence referendum, as the Tory leadership race enters its next phase, with Conservative MPs due to vote in the second ballot.
Writing in today's Sunday Times, Mr Hunt gave his backing to Ruth Davidson and said he "will fight for the union" but indicated as Prime Minister he would be prepared to grant a section 30 order and discuss the wording of any second independence vote if the SNP met three tests.
He also criticised Nicola Sturgeon's stance during the Brexit process, saying she "failed to listen" to Scottish businesses who backed the EU withdrawal agreement.
But the First Minister reacted on Twitter, branding his comments as "Tory arrogance".
Hunt's rivals for the Tory leadership have ruled out the possibility of a second independence referendum, but Mr Hunt says the SNP needs to win an overall majority in the 2021 Holyrood election, lay out clear currency plans and declare there would be no "wildcat" vote.
He writes: "The SNP said they could only propose another referendum if a majority supported independence. The Scottish people will hold them to that test. And from the granting of a Section 30 order and the negotiations on the wording I will fight for the Union every step of the way."
He says as Prime Minister he would put "three stringent challenges" to the SNP "before they hold another divisive referendum".
He lists the SNP needing to win an "outright majority in the 2021 Scottish Parliament elections"; on currency he says: "people's mortgages and pensions are at stake. Sturgeon must provide a clear timeline on her proposal to ditch the pound"; and he says "the SNP should rule out any suggestion of a wildcat referendum. People need to know they can trust the political process".
He adds: "By supporting the Scottish people to pose these challenges prior to a section 30 order my government would lead the way on opposing independence.
"I want to use the economic engine of the UK government to drive support for the Union. The SNP can and will be halted in their tracks because the truth is, workers across the UK want the same thing: a prime minister with an economic vision to deliver higher wages, better jobs and more money for our vital public services."
On Twitter Nicola Sturgeon responded saying she "*sits back and allows Tory arrogance and incompetence to further strengthen support for independence*. The Scottish people will decide Scotland’s future, no-one else."
Posting a link to the SNP 2016 manifesto which included a commitment to holding a referendum if there was a "material change" such as Scotland being "taken out of the EU", she added: "There’s majority support for this in @ScotParl. Tories standing in the way of the will of the Scottish people never ends well for them. This time will be no different. #indyref"