Friends of Brexit Secretary David Davis fear he is on the brink of resigning over the bombshell Brexit letter sent by Boris Johnson and Michael Gove to Theresa May.

The letter setting out their blueprint for a post-EU Britain was kept secret from the rest of the Cabinet – including Mr Davis – until it was revealed in last week’s Mail on Sunday.

Now we can disclose that the missive effectively calls for Mr Davis to be sidelined by a new, non-elected ‘Brexit Tsar’. Mr Davis’s allies say he is ‘deeply frustrated’ by the way Mr Johnson and Mr Gove went behind his back to hand-deliver the letter to the Prime Minister’s chief of staff Gavin Barwell. Last night, No 10 added to his humiliation by failing to deny that the Brexit Secretary had still not seen a copy of the letter.

Friends of Brexit Secretary David Davis fear he is on the brink of resigning over the bombshell Brexit letter sent by Boris Johnson and Michael Gove to Theresa May

In the full letter, Foreign Secretary Mr Johnson and Environment Secretary Mr Gove lend weight to claims that some Brexiteers want Mrs May to turn the UK into the ‘Singapore of Europe’ by slashing tax and red tape.

And they say she must prepare for crashing out of the EU after a ‘total breakdown’ in Brexit talks.

The rebel pair flex their muscles by saying she should force all Cabinet Ministers to ‘submit to a process’ to prove they are enforcing a ‘hard Brexit’ blueprint – including recruiting thousands of extra customs and immigration officers. The 2,500-word letter portrays Mrs May as an enfeebled PM forced to take orders from the two men who brought down her predecessor David Cameron.

In addition to sidelining Mr Davis, they also take aim at Chancellor Philip Hammond, whose supporters were furious when we revealed how Mr Johnson and Mr Gove accused him of failing to show ‘sufficient energy’ to prepare for a ‘no-deal’ outcome with the EU.

The new disclosures come as Mr Davis enters the final stages of crunch talks with the EU over the size of our divorce bill – and before another week of Commons clashes over the EU Withdrawal Bill, with 20 Tory MPs threatening to rebel over Mrs May’s plans to fix in law the date of Britain’s departure.

Former Chancellor Kenneth Clarke claimed last week that the fixed date move was a ‘sop’ to Mr Johnson and Mr Gove in the wake of their letter, which told the PM to announce a date for the end of the UK’s ‘transition’ out of the EU.

The letter will revive the debate over whether Britain could use Brexit to become a low-tax, low- regulation ‘Singapore-style’ economy. It says: ‘We may choose to remain identical to the EU – or we may embrace a vision more aligned with pro-competitive regulation. Other countries must know this choice is in our hands – and they must know it on day one.’

An overhaul of science and technology rules will ‘give the UK big advantages over EU members,’ they argue. ‘Our tax system is hugely complex after the long years of Labour government. There will be huge savings from reducing the burden of compliance.’

The letter setting out their blueprint for a post-EU Britain was kept secret from the rest of the Cabinet – including Mr Davis (pictured) – until it was revealed in last week’s Mail on Sunday

Supporters of the idea say it would create an economic boom – but critics say it would undermine workers’ rights.

The Ministers attempt to address this criticism by saying that it is ‘not about deregulating’ but about ‘regulating better’, with a ‘vision more aligned with pro-competitive regulation’.

The authors tell Mrs May to crack down on Ministers who refuse to support the proposal and ‘identify individual Cabinet Ministers and submit them to a process that produces action’.

They say Mrs May should seek a Free Trade Agreement with the EU: they do not specify which model they prefer, though they reject the so-called Norway model, supported by Mr Hammond, under which the UK would retain access to the single market while still being bound by its regulations.

The authors tell Mrs May to crack down on Ministers who refuse to support the proposal and ‘identify individual Cabinet Ministers and submit them to a process that produces action’

The letter also spells out the risk of the Brexit talks collapsing before Britain leaves in 2019 – and says there may be no trade deal even by the time the so-called transition period, scheduled to end in 2021, is over.

‘We may not be able to reach agreement on the final deal during the implementation period or even know by March 2019 if an agreement is going to be possible or not,’ write Mr Johnson and Mr Gove.

‘The worst case is a total breakdown in March 2019, but even if we avoid that we may still have a “no-deal” outcome at the end of the transition period.’

They fear chaos at ports and airports. ‘We are most concerned about customs rules where the UK must be in a position to charge import duties and conduct checks on EU goods. This will require upgrading facilities at ports and recruiting thousands of customs officers. Similarly, we must arrange to implement immigration controls in the event of no deal.’

The letter also spells out the risk of the Brexit talks collapsing before Britain leaves in 2019 – and says there may be no trade deal even by the time the so-called transition period, scheduled to end in 2021, is over

They claim mandarins are incapable of implementing a ‘hard Brexit’ and must be replaced by Ministers backed by ‘strong’ political apparatchiks and the urgent appointment of a new, non-elected ‘Brexit Tsar’ in charge of an Implementation Task Force (ITF).

‘The ITF leader should have direct authority to mobilise Whitehall on Brexit preparation issues and brief the Cabinet.’

This newspaper has been told Tory Brexit leaders want Matthew Elliott, former chief executive of Vote Leave, and now a senior fellow with pro-Brexit think-tank the Legatum Institute, to be the tsar.

The letter states that Mrs May must not allow the European Court of Justice to have jurisdiction over new EU rules in the UK during the transition.

They write: ‘There should be no question of the UK implementing new EU rules during this period – or ECJ jurisdiction on any new rules. Clarifying that in the minds of colleagues who have not yet internalised that logic would help.’

This newspaper has been told Tory Brexit leaders want Matthew Elliott, former chief executive of Vote Leave, and now a senior fellow with pro-Brexit think-tank the Legatum Institute, to be the tsar

This is at odds with Mr Davis, who said on Friday that Britain will remain within the jurisdiction of the EU’s Court of Justice immediately after Brexit in March 2019, and it would not be phased out until the end of the two-year transition period.

This angered the influential ‘hard Brexit’ Tory pundit Charles Moore, who yesterday accused Mr Davis of letting EU negotiator Michel Barnier run rings round him – and he also took a swipe at recent claims about Mr Davis’s drinking.

Mr Moore wrote: ‘It is time to get annoyed with Mr Davis. For ages now, he has been flying from meeting to meeting, speaking at dinner after dinner, staying late at party after party, encouraging his bonhomous reassurance to be favourably contrasted with Mrs May’s anxious gloom.

‘Michel Barnier has the right idea – a grave expression, a guardedly polite manner and the tenacious pursuit of what he wants. Unfortunately, he’s on the other side.’

A spokesman for Mr Davis said: ‘It is completely wrong that he is considering resigning and anyone pushing this nonsense in order to undermine Brexit is going to be sorely disappointed.’

Downing Street refused to comment.