THERESA May was last night warned not to bypass Scotland over the EU, after it was reported she might trigger Brexit without seeking parliamentary approval.

The SNP reminded the Prime Minister that, at her first official meeting with Nicola Sturgeon, she had promised a “UK-wide approach” before starting the Article 50 withdrawal process.

The party said it would "resist any attempt to bypass parliament and drag Scotland out against its will".

It emerged yesterday that MPs from all parts of the UK could be sidelined by May, who has received legal advice than she can proceed with Brexit without a vote at Westminster.

The Daily Telegraph reported government lawyers had told the PM she has the executive power to invoke Article 50, denying pro-Remain MPs a chance to block Brexit.

A Downing Street source said the report was “speculation” but did not reject it out of hand and said May was "committed to delivering on the verdict the public gave".

Most MPs and peers were in favour of the UK remaining before the June 23 vote to Leave, and there have been calls from Remain campaigners for parliament to frustrate or even prevent Brexit.

Labour leadership hopeful Owen Smith last week said he wanted parliament “to block any attempt to invoke Article 50 until Theresa May commits to a second referendum or a general election on whatever EU exit emerges at the end of the process”.

LibDem leader Tim Farron has also said his party will stand at the next general election on a platform of derailing Brexit and staying in the EU, regardless of the 52-48 vote to Leave.

A group of lawyers will go to the High Court in October in an attempt to force May to hold a parliamentary vote by arguing Article 50 cannot be triggered until after the repeal of the European Communities Act of 1972 – something MPs would have to approve.

However government lawyers are expected to say May can use the royal prerogative to start the two-year withdrawal, paving the way for Brexit in 2019.

SNP Europe spokesman Stephen Gethins said May seemed intent on “ploughing ahead with a hard Brexit regardless of the damage” and the 62-38 vote in Scotland to Remain.

He said: “Just two months ago the Prime Minister pledged to govern in the interests of all nations in the UK – if that was a genuine commitment then the UK government must respect the clear wishes of the people of Scotland who voted overwhelmingly to remain in the EU.

“The SNP is focused on protecting Scotland's relationship with Europe, and the Scottish Government must be at the centre of negotiations.

“SNP parliamentarians would not vote for any proposal that would take Scotland out of the EU and we will resist any attempt to bypass parliament and drag Scotland out against its will."

Despite pro-Brexit Tory MPs pushing for Article 50 to be triggered in the New Year, the former head of the civil service yesterday said Brexit was not inevitable and Britain might end up remaining part of a changed, looser European Union.

Ex-cabinet secretary Gus O'Donnell, who was in charge of the Civil Service between 2005 and 2011 and is now a crossbench peer, told The Times: “Lots of people will say, 'We've had the referendum, we've decided to go out, so that's it, it's all over'. But it very much depends what happens to public opinion and whether the EU changes before then.

"It might be that the broader, more loosely aligned group, is something that the UK is happy being a member of."

He later told BBC Radio 4 the probability of Brexit being called off was “very low”, but also advised against rushing into Article 50, as government had to plan exactly what it wanted from the subsequent negotiations.

He said elections in France and Germany next year mean "it is not even clear which leaders our Prime Minister will be negotiating with, so I don't think there's any great rush to do it".

Last week, the Sunday Herald revealed O’Donnell’s successor as head of the civil service, Lord Kerslake, believed Brexit would take “at least five years” because of its complexity.

Meanwhile, former Tory leader Iain Duncan Smith suggested Britain could rely on bare bones World Trade Organisation (WTO) rules to trade with the EU following Brexit.

Identified as the worst case scenario by the Scottish Government last week, WTO rules would put the UK outside the EU single market, with tariffs imposed on UK exports to Europe.

However they would give the UK full control of the movement of people from the EU.

Mr Duncan Smith told Radio 4 that European countries including Germany are eager to negotiate a trade agreement with the UK.

He said: "I think we would like to have, and I think it would be in the interests of the European Union even more than it might be for the UK, to maintain a trading relationship with the UK.

"Bearing in mind that we will anyway have access to the marketplace under WTO rules, so the question really is – do we want more preferential arrangements than that?"

Asked if he would be happy with WTO rules, he said: "I'm saying that's the extent of where you could be and you'd still get access to the marketplace.

"I've already made it clear that my personal view is we should not seek to remain a member of the customs union nor necessarily remain a full member of the single market, because that would entail putting yourself yet again under the rule of European law.

"And that was one of the key areas that the British public voted for in the process of taking back control."