Boris Johnson has been told to ‘obey the law’ over Brexit in an extraordinary warning issued by his own justice secretary.

But he also revealed his concerns about the prime minister, by tweeting: “We have spoken over the past 24 hours regarding the importance of the Rule of Law, which I as Lord Chancellor have taken an oath to uphold.”

The pro-EU justice secretary is one of four cabinet ministers on ‘resignation watch’, as No 10 ratchets up the rhetoric in a Brexit showdown heading for the courts.

Dominic Raab, the foreign secretary, revealed the government hopes to win a legal fight to defeat the legislation requiring it to seek an Article 50 extension, designed to avoid a no-deal Brexit on 31 October.

Vowing to “test to the limit” what the new law demands, Mr Raab said: “We will look very carefully, legally at what it requires and what it doesn’t require.”

Protesters take to streets after Boris Johnson suspends parliament Show all 11 1 /11 Protesters take to streets after Boris Johnson suspends parliament Protesters take to streets after Boris Johnson suspends parliament Anti Brexit protesters march on Whitehall EPA Protesters take to streets after Boris Johnson suspends parliament Many of the protesters carried placards AP Protesters take to streets after Boris Johnson suspends parliament A demonstrator wears a Boris Johnson mask during a London rally AFP/Getty Images Protesters take to streets after Boris Johnson suspends parliament An anti-Brexit demonstrator wields an EU flag during a London rally. AFP/Getty Images Protesters take to streets after Boris Johnson suspends parliament The protesters also marched to Trafalgar Square, where they blocked traffic AP Protesters take to streets after Boris Johnson suspends parliament John McDonnell addressed crowds in Whitehall. PA Protesters take to streets after Boris Johnson suspends parliament A small group of Brexit supporters attempted to stage a counter protest in central London. AP Protesters take to streets after Boris Johnson suspends parliament Three people were arrested in London, including a woman who sat on the road near Trafalgar Square. AP Protesters take to streets after Boris Johnson suspends parliament In Oxford, locals and students gathered outside Balliol College, Boris Johnson's alma mater, to protest. PA Protesters take to streets after Boris Johnson suspends parliament Thousands of people gather in central Glasgow to protest against the prime minister. AFP/Getty Images Protesters take to streets after Boris Johnson suspends parliament Jeremy Corbyn spoke to protesters in central Glasgow. Getty

And, pointing to the failed legal actions to stop parliament being suspended, he told Sky’s Sophy Ridge programme: “We had two legal challenges last week and we won both of those.”

Nicky Morgan, the culture secretary, Matt Hancock, the health secretary, and Julian Smith, the Northern Ireland secretary, are the other three cabinet ministers most likely to follow Ms Rudd out of the door.

But all three, like Mr Buckland, made clear they are staying put for now, despite Ms Rudd accusing Mr Johnson of misleading the country by not really seeking a fresh Brexit deal.

In his tweet, Mr Buckland wrote: “Speculation about my future is wide of the mark. I fully support the prime minister and will continue to serve in his cabinet.”

It now appears clear that Mr Johnson’s ‘plan B’ is not to break the law – to be given royal assent on Monday – but not to comply either.

Instead, the prime minister will defy the order to delay Brexit to manufacture a case before the Supreme Court – with the Halloween deadline for crashing out just days away.

Mr Raab has dismissed a warning by a former director of public prosecutions that the prime minister is heading for jail if he flouts the law as “ridiculous”.

But Shami Chakrabarti, Labour’s shadow attorney general, has condemned the strategy, saying: “Is that what we say to our kids? Is that what we say to vulnerable kids? It's irresponsible and elitist.”

The foreign secretary also claimed the government had failed to put forward new proposals to secure a Brexit deal for fear the EU would leak them.