BORIS Johnson is under mounting pressure as Prime Minister after senior Scottish Tories questioned key planks of his Brexit strategy.

Interim party leader Jackson Carlaw told The Herald on Sunday that he wants to find a way to readmit the 21 MPs who had the Tory whip withdrawn for defying Johnson.

MSP Adam Tomkins, the party spokesperson on constitutional issues, issued a coded warning to Johnson not to defy a new law that would delay Brexit. It comes as a group of MPs are reportedly poised to challenge the Government if the law is disobeyed.

And fellow MSP Donald Cameron, who is his party's policy chief, said: "Upholding the rule of law is a basic principle of our democracy and one which we should all respect."

Johnson, who took over from Theresa May as Prime Minister in July, endured a nightmare week after his Brexit plan was effectively derailed.

The Uxbridge and South Ruislip MP won the Tory leadership on a promise to leave the EU by the end of October, but a series of votes in the House of Commons have threatened his “do or die” commitment.

The Opposition parties united to take control of the parliamentary agenda, following which legislation was passed to compel the Prime Minister to seek a delay to Brexit.

A miserable week for Johnson was capped after he failed to engineer a general election, which he says is required to break the logjam.

However, it emerged yesterday that Johnson wrote to Tory members on Friday and suggested he could ignore the new law.

He informed party supporters: "They just passed a law that would force me to beg Brussels for an extension to the Brexit deadline. This is something I will never do."

The BBC reported on Saturday that cross-party MPs, including expelled Conservatives, had sought legal advice and were preparing to go to court "to compel Mr Johnson to seek a delay".

Asked if he would obey the new law's demand for him to write to EU leaders requesting more time, Johnson said: "I will not. I don't want a delay."

However, the threat of law breaking has alarmed Tory moderates. Tomkins, a Glasgow MSP who is also a professor of public law, tweeted yesterday:

“Irrespective of what we think about Brexit, or the PM, surely we can all agree on one fundamental principle: the government is bound to obey the law.

“If the law compels the PM to act in a certain way, and if the PM refuses so to act, he has only one option: to resign his ministry. It really is as simple as that.”

Stephen Kerr, the Tory MP for Stirling, agreed:

“You cannot pass laws and then pick and choose which laws to obey. Whether you are an organisation or a Government, we all have to obey the law. It is a black and white issue.”

Carlaw, who is acting as temporary leader in the wake of Ruth Davidson’s shock resignation, piled further pressure on Johnson.

After the Government’s first defeat last week, 21 MPs, including well known Conservatives such as Ken Clarke, Rory Stewart and Sir Nicholas Soames, were kicked out.

The move was widely criticised and led to Cabinet Minister Michael Gove making representations to Johnson to change his mind.

Carlaw told this newspaper: "The Conservative party works best when it's united. Every Conservative, including me, hopes a way can be found for these distinguished parliamentarians to return to the fold."

Meanwhile David Lidington, the de facto deputy prime minister when May was in Downing Street, said it would set a "dangerous precedent" if Johnson opted to disobey the law.

Lidington, a former Europe minister, told BBC Radio 4's Today programme: "It is such a fundamental principle that we are governed by the rule of law that I hope no party would question it.

"Defying any particular law sets a really dangerous precedent."

The former Cabinet minister said Johnson had convinced him that he still wanted to strike a deal with the EU.

However, former Conservative leader Iain Duncan Smith encouraged Johnson to break the law, saying he would be seen as a Brexit "martyr" if judges opted to put him jail for breaching Parliament's terms.

Duncan Smith told a newspaper: "This is about Parliament versus the people. Boris Johnson is on the side of the people who voted to leave the EU.”

The new law will rule out an early general election before the European Council summit on October 17, as Labour and other opposition parties want the threat of leaving the EU on Halloween to have expired before agreeing to a fresh poll.

Labour, the Liberal Democrats, SNP and Plaid Cymru met on Friday and agreed to block the PM's election request when it is put to the House of Commons again on Monday.

A similar motion was defeated by MPs on Wednesday, failing to make the two-thirds threshold needed to dissolve Parliament.