FOUR Scottish Conservatives elected to the Commons last year are among the 60 Eurosceptic Tory MPs who have warned the Prime Minister her government will collapse unless she shelves her preferred plan for a post-Brexit customs system.

Ross Thomson, Stephen Kerr, Colin Clark and Alister Jack are all members of the European Research Group (ERG), led by arch-Eurosceptic Jacob Rees-Mogg, that has sent Theresa May a 30-page report opposing her plan for a “customs partnership”.

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One ERG source told a daily newspaper: “We have swallowed everything so far – but this is it. If they don’t have confidence in Brexit we don’t have confidence in them. The Prime Minister will not have a majority if she does not kill off the NCP [New Customs Partnership].”

Rees told the paper the PM’s model is flawed, “deeply unsatisfactory” and would keep the UK tied to the single market. He said the letter sent to May was not an ultimatum from Eurosceptic MPs, but he would not commit to backing the plan should it be put to Parliament as part of the final Brexit agreement.

Tommy Sheppard, the SNP MP for Edinburgh East, said Ruth Davidson was not in control of the four Scottish Tory Eurosceptics, who he said appeared to be taking instruction from Rees-Mogg and not the Scottish Conservative leader.

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“This group of British nationalists, who long for the return of days of Empire and for the time when the map of the world was mostly pink, must be an acute embarrassment to Ruth Davidson,” he said.

“It’s pretty clear the first allegiance of this group of Scottish Tory MPs is to the hard right of the English Conservative party. They are demonstrating the number one problem about the Tories – no matter how hard Ruth Davidson tries to sugar coat their image – their true nature lurks not far beneath the surface.”

The same four MPs hit the headlines in February when they backed a letter to May setting out their “red lines” and making clear what she must do to retain their loyalty. Clark, Jack and Kerr were among the 62 Tory MPs who signed the letter, demanding the UK must retain “full regulatory autonomy” after it leaves the EU and said it must be free to sign trade agreements with other countries during the transition period.

The letter, signed by four former Cabinet ministers and nine other former ministers, was the most public intervention to date by the ERG group, without whose support May is unable to govern. Thomson supported the letter but his name did not appear because of an administrative error.

It’s demands were designed to tilt the argument in favour of a harder Brexit and was described as a “ransom note” by Tory Remainer Nicky Morgan.

The intervention was timed to coincide with a meeting of the Brexit war cabinet and designed to put pressure on May to drop her preferred option of a so-called hybrid customs model, under which the UK would collect EU import tariffs on behalf of Brussels and then refund any companies when the goods’ end use is not in the EU27.

Brexiteers believe the model would open the door to remaining in a customs union, while the plan’s backers believe it would solve the Irish border question and still allow Britain to strike trade deals. But Rees-Mogg and the ERG claim the plan would “festoon the entire economy with burdensome controls” and cripple the ability of the UK to negotiate trade deals.

Tory sources said the ERG group had threatened to withdraw support for the government if the plan was pushed through. But Rees-Mogg said the report should not be seen as a threat to May’s position.

“I am neither John Wayne nor James Stewart,” he said.

“That is not the way we would behave towards the Prime Minister. There is no question of this being an ultimatum.”

He added that the customs plan would not deliver on the party’s manifesto or other commitments by the PM. A spokeswoman for the Scottish Conservative MPs did not respond for a request for a comment.