RUTH Davidson has warned Theresa May she cannot rely on the support of all 13 Scots Tory MPs in forthcoming votes on Brexit and trade legislation.

The Scottish Tory leader said her MPs were not “drones”, but “sentient beings” who would make up their own minds and vote according to their principles.

She made clear any support for the Prime Minister was conditional, and the “the grounds on which we will support her” involved the kind of Brexit deal she negotiated.

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Last month more than 60 Tory MPs in the European Research Group led by Brexiter Jacob Rees-Mogg signed a letter to Mrs May demanding a hard Brexit, with a clean break from the EU single market and customs union.

If necessary, the UK could fall back on World Trade Organisation (WTO) rules in a no deal Brexit, it said.

It was signed by three Scots Tory MPs - Colin Clark, Alister Jack, and Stephen Kerr - while a fourth, Ross Thomson, said he would have signed but for an “admin error”.

On BBC Radio Scotland, Ms Davidson was asked whether she could guarantee her MPs would vote with Mrs May on Brexit-related trade legislation, which some Tory and Labour MPs intend to amend to ensure some form of customs union between the UK and EU.

Ms Davidson said: "We will support her if she fulfils her principles, which I expect her to do."

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She said those included maximising trade, making the UK stronger, and rejecting an EU proposal for Northern Ireland to have a separate status from the rest of the country.

Asked why she was setting conditions on her support, Ms Davidson said: “I’m a thinking and sentient human being and I would expect all elected representatives to have principles to which they hold.

“We’re not the SNP. We don’t believe our elected representatives should be drones that never question and never use their brains. "We actually expect and require our elected representatives to think and be sentient beings for themselves.

“Beyond [Scottish Secretary] David Mundell, none of the [Scots] Conservative MPs hold ministerial office. They’re not bound by cabinet responsibility. This is a very big decision.

“I expect them to take it seriously. They are. We have together agreed what the principles are for the Scottish Conservatives. We expect the Prime Minister to uphold that.”

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Ms Davidson also backed the UK Government mounting a legal challenge at the UK Supreme Court if MSPs pass the SNP’s alternative Brexit Bill.

Holyrood’s Presiding Officer, Ken Macintosh, this week said he believed the continuity bill was beyond the parliament’s legislative competence as it was incompatible with EU law.

However the Lord Advocate, James Wolffe QC, the government’s most senior law officer, has said the Bill is competent and does not breach EU law.

If the Bill is passed - and SNP ministers say it would be withdrawn if London agree a deal on devolved powers after Brexit - the dispute is likely to be decided by the UK Supreme Court.

Asked about a Guardian report that UK ministers intended to challenge the Bill immediately if it was passed, Ms Davidson said: “Lawmakers like politicians have a duty to respect the law, and I think that if this in contravention of that, then, absolutely, it is incumbent upon us to test that.”

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Referring to a failed intervention over Article 50, she added: “The Lord Advocate has, with respect, gone to Supreme Court before on constitutional issues, on Brexit, and lost.”

SNP MSP George Adam said: "It's clearer than ever that Ruth Davidson is being led by the hard-line Brexiteers in her party, rather than standing up for Scotland's interests.

"The Tories now think they can do anything to Scotland and get away with it. Ruth Davidson has abandoned her previous support not only for staying in the EU but the single market too - which she once said was the 'overriding priority'."