Theresa May faced multiple warnings that her plans for Brexit risk descending into chaos, a fresh strategy to neuter Parliamentary opposition has revealed.

Downing Street found itself warding off powerful criticisms of Whitehall’s readiness for the huge task – even as Boris Johnson appeared to let slip a key aspect of the exit plan.

Meanwhile, in a dramatic intervention, a Supreme Court judge suggested “comprehensive” legislation may be required to start the withdrawal process, potentially delaying the move for two years.

In a day of Brexit twists and turns at Westminster:

* The Institute for Government (IfG) described it as an “existential threat” to cash-starved Government departments, with the process appearing “chaotic and dysfunctional”.

* Mr Johnson, the Foreign Secretary, said Britain will “probably” leave the EU’s customs union – even as No.10 insisted no decision had been taken.

* A short “three line Bill” to kick-start Brexit has been drawn up by the Government to prevent Parliament holding it up, the BBC reported.

* Lady Hale – one of 11 justices who will hear the Government’s appeal over the right to trigger Brexit – said it may require full replacement of the 1972 Act that took Britain into the EU, an enormously complex process.

* No.10 launched an attack on accountants Deloitte, after its leaked memo claimed Cabinet splits meant a proper plan for Brexit was as much as six months away.

* The Institute for Fiscal Studies predicted that sterling’s collapse will drive up the cost of running a car by 5.1 per cent and make clothes and footwear (up 3.1 per cent) and food (up 2.9 per cent) more costly.

The IfG blog – based on conversations with key figures inside and outside Whitehall – criticised Ms May’s “secretive approach”, which had left senior civil servants knowing “little more than the general public”.

The IFG's Joe Owen wrote: “Silence is not a strategy. Failure to reveal the Government's plan to reach a negotiating position is eroding confidence among business and investors.”

The verdict had echoes of the Deloitte paper, obtained by The Times, which said 30,000 extra civil servants could be needed to deliver Brexit, with “well over 500 projects” being undertaken.

The Prime Minister’s spokeswoman accused Deloitte of “touting for business”, insisting the “unsolicited document” had not been commissioned by the Government, or seen by Ms May.

No.10 was helped by a later Deloitte statement, which said: “This was a note intended primarily for internal audiences.

“It was not commissioned by the Cabinet Office, nor any other Government department, and represents a view of the task facing Whitehall. This work was conducted without access to Number 10 or input from any other Government departments.”

Mr Johnson’s intervention came in a colourful interview with a Czech newspaper, in which he also claimed it was “bollocks” to suggest free movement of people is one of the EU’s founding principles.

He said: “Probably, we will need to leave the customs union, but this is a question which will be dealt with in the negotiations.” No.10 insisted that “a decision has not been taken yet”.

The spokeswoman also dismissed as “speculation” the report that a brief Bill - “bombproof” to amendments - will be pushed through the Commons in two weeks, in January.

Meanwhile, Iain Duncan Smith reacted with fury to Lady Hale’s suggestion that it could be legally impossible to kick-start Brexit next year, because repeal of the 1972 Act may not be completed until 2019.