Boris Johnson has hinted he plans to cling to office even if the Supreme Court decides he misled the Queen when he shut down parliament, in Tuesday’s momentous ruling.

Asked if such a finding – that he concealed his true motive was to silence parliament over Brexit – would make his position “untenable”, the prime minister replied: “No”.

Until now, Mr Johnson has refused to comment on what he will do if the government loses the Supreme Court action, in a judgment to be delivered at 10.30am.

It will still be dark in New York City when he hears the verdict on accusations that he acted unlawfully, with an “abuse of power”, by proroguing parliament for five weeks.

Most legal experts appear to believe the 11 justices will find against the government, plunging Britain into an unprecedented constitutional crisis.

Supreme Court protests: Boris Johnson's prorogation legal or not? Show all 15 1 /15 Supreme Court protests: Boris Johnson's prorogation legal or not? Supreme Court protests: Boris Johnson's prorogation legal or not? Protesters gather outside The Supreme Court ahead of a hearing on the legality of Boris Johnson's proroguing of Parliament Getty Supreme Court protests: Boris Johnson's prorogation legal or not? The supreme court justices will sit as a panel of 11 judges to hear the challenge, brought by campaigner Gina Miller, that the Prime Minister acted unlawfully when he advised the Queen to suspend parliament AP Supreme Court protests: Boris Johnson's prorogation legal or not? A woman protests outside the Supreme Court Reuters Supreme Court protests: Boris Johnson's prorogation legal or not? The PM sent lawmakers home on 9 September until 14 October, just over two weeks before the UK is due to leave the European Union AP Supreme Court protests: Boris Johnson's prorogation legal or not? Gina Miller arriving AP Supreme Court protests: Boris Johnson's prorogation legal or not? People protest outside the Supreme Court Reuters Supreme Court protests: Boris Johnson's prorogation legal or not? Boris Johnson likened himself to the Incredible Hulk days previous as he promised to ignore the Commons legislation ordering him to delay Brexit if negotiations break down PA Supreme Court protests: Boris Johnson's prorogation legal or not? Officials bring in documents Reuters Supreme Court protests: Boris Johnson's prorogation legal or not? Protesters hold banners outside the Supreme Court in London, Tuesday Sept. 17, 2019. The Supreme Court is set to decide whether Prime Minister Boris Johnson broke the law when he suspended Parliament on Sept. 9, sending lawmakers home until Oct. 14 â€” just over two weeks before the U.K. is due to leave the European Union. (AP Photo/Matt Dunham) Matt Dunham AP Supreme Court protests: Boris Johnson's prorogation legal or not? Protesters hold banners outside the Supreme Court in London, Tuesday Sept. 17, 2019. The Supreme Court is set to decide whether Prime Minister Boris Johnson broke the law when he suspended Parliament on Sept. 9, sending lawmakers home until Oct. 14 â€” just over two weeks before the U.K. is due to leave the European Union. (AP Photo/Matt Dunham) Matt Dunham AP Supreme Court protests: Boris Johnson's prorogation legal or not? Protesters hold banners outside the Supreme Court in London, Tuesday Sept. 17, 2019. The Supreme Court is set to decide whether Prime Minister Boris Johnson broke the law when he suspended Parliament on Sept. 9, sending lawmakers home until Oct. 14 â€” just over two weeks before the U.K. is due to leave the European Union. (AP Photo/Matt Dunham) Matt Dunham AP Supreme Court protests: Boris Johnson's prorogation legal or not? People protest outside the Supreme Court of the United Kingdom during a hearing on Prime Minister Boris Johnson's decision to prorogue parliament, in London, Britain September 17, 2019. REUTERS/Phil Noble PHIL NOBLE Reuters Supreme Court protests: Boris Johnson's prorogation legal or not? Protesters hold banners outside the Supreme Court in London, Tuesday Sept. 17, 2019. The Supreme Court is set to decide whether Prime Minister Boris Johnson broke the law when he suspended Parliament on Sept. 9, sending lawmakers home until Oct. 14 â€” just over two weeks before the U.K. is due to leave the European Union. (AP Photo/Matt Dunham) Matt Dunham AP Supreme Court protests: Boris Johnson's prorogation legal or not? Joanna Cherry of the Scottish National Party leaves the Supreme Court Reuters Supreme Court protests: Boris Johnson's prorogation legal or not? Demonstrators protest outside the Supreme court in central London on the first day of the hearing into the decision by the government to prorogue parliament on September 17, 2019. (Photo by Tolga Akmen / AFP)TOLGA AKMEN/AFP/Getty Images TOLGA AKMEN AFP/Getty

The prime minister’s case is that the shut-down, from 9 September until 14 October, was to allow a new legislative agenda in a Queen’s Speech, after the longest session since the 17th century.

The High Court in London ruled in favour of the government, that the prorogation was “purely political” and not a matter for the courts.

But, in Scotland, a cross-party group of MPs and peers won a ruling that the decision was unlawful because it was “motivated by the improper purpose of stymieing parliament”.

Most toxically for Mr Johnson, it found his “advice” to the Queen was untrue – noting the apparent refusal of either ministers or senior civil servants to provide sworn witness statements.

The Supreme Court heard appeals arising out of those two separate legal challenges, providing an ultimate ruling on the very different conclusions in England and Scotland.

The mood music appeared troubling for the government after the judges spent a large portion of their time exploring possible remedies – what they might determine must happen if they find against the prime minister.

The government’s lawyer also came in for forensic questioning, suggesting doubts about the arguments being put forward, and the fact the verdict was delayed from last Thursday also sent alarm bells ringing.

Dominic Grieve, the former Conservative attorney general, said Mr Johnson would have to resign if the Supreme Court determined that he had misled the Queen.

But some observers believe the court might frame its verdict around whether the suspension sought to block the passage of legislation through parliament, which is potentially less explosive.

In a separate interview, Mr Johnson declined to discuss what happens next, saying: “Let’s wait to hear what the justices say.