A CUSTOMS proposal being pushed by Boris Johnson and backed by four Scottish Tory MPs would damage hundreds of thousands of companies, min isters have been told.

The Treasury analysis warned that the “maximum facilitation” option supported by hardline Brexiteers such as Ross Thomson, Colin Clark, Stephen Kerr and Alister Jack would shave 1.8% off GDP in the long term.

The leak of details about the damning report is the latest development in the row between Theresa May’s divided Cabinet over future customs ties with the European Union.

The Foreign Secretary this week dismissed the Prime Minister’s preferred option – a “customs partnership” – as “crazy”.

Meanwhile, there were reports Brexit Secretary David Davis was prepared to storm out of the Cabinet if the Prime Minister did not drop the plan in favour of the so-called maximum facilitation model championed by Brexiteers.

Reports yesterday revealed ministers have been briefed on the potential economic impacts of the “max fac” plan – and warned that 145,000 companies that rely on trading with the EU would be hit by new barriers to selling products there.

The assessment, prepared by the Treasury, also warns that the Brexiteers’ favoured option – which relies on technological fixes to keep customs friction to a minimum – would leave the UK economy 1.8% worse off in the long run.

According to the Buzz News website, just 40 people in the Government have been given access to the sensitive presentation, which was shown to the Cabinet Brexit sub-committee last week.

The Prime Minister on Wednesday insisted that both options remained under consideration despite the war Cabinet last week siding against the partnership plan, which would keep much closer customs ties with the EU and see the UK collect tariffs on behalf of Brussels.

Clark, Kerr and Jack were among a group of 62 Eurosceptic Tory backbenchers who signed a letter – described by a critic in February as a ransom note – to May calling for a clean break with the EU.

The letter said that the UK must gain full “regulatory autonomy” after Brexit, and must not be stopped from negotiating trade deals with other countries. It was sent by the European Research Group headed by leading Brexiteer Jacob Rees-Mogg.

A fourth Scottish Conservative, Aberdeen South MP Ross Thomson, later said he “wholeheartedly backs” the letter, but that his name was not on it due to an “admin error”.

Jack, the MP for Dumfries and Galloway, told BBC Scotland at the time that the aim of the letter was to ensure that the Prime Minister’s promises in her Lancaster House speech a year ago will be delivered, and that the UK will leave both the single market and the customs union.

Johnson has insisted his outspoken intervention was “completely in conformity with government policy” but only because the UK still does not have a policy on the issue.

May is considering making a public intervention on the issue next week in a bid to break the Cabinet deadlock over which customs policy to back. The EU has indicated that both options are not acceptable.