The Brexit legal battle over Boris Johnson's decision to shut down Parliament for five weeks starts in the Supreme Court today where 11 judges will decide if the Prime Minister broke the law and misled the Queen.
Britain's highest court will sit for the next three days where arch-remainers including Gina Miller and Sir John Major will argue that Mr Johnson is trying to 'stymy' scrutiny of his Brexit policy.
But the Prime Minister said last night it was 'claptrap' that he had stopped MPs having their say on Brexit and said a Queen's Speech was required to end the longest Parliamentary session since the English civil war ended in 1651.
Over the next three days the Supreme Court in London will hear appeals from two separate challenges brought in England and Scotland to the prorogation of Parliament and will return verdicts in both by next week.
On the same day last week an Edinburgh court ruled it was unlawful while the High Court in London disagreed and said the decision was 'purely political'.
What the 11 Supreme Court judges finally decide will have huge implications for Boris Johnson’s premiership and could even influence whether Britain manages to leave the EU or not.
The justices, led by president Lady Hale, will grapple with whether the PM's decision about the length of the prorogation is not a matter for the courts - or unlawful.
Boris Johnson's Brexit strategy could be saved or completely unravel if the Supreme Court sides with remainers including Gina Miller who say prorogation of Parliament was unlawful
Mr Johnson says the five-week suspension is to allow the Government to set out a new legislative agenda in a Queen's Speech when MPs return to Parliament on October 14.
John Major will take the stand in the case against Boris Johnson's decision to suspend Parliament, it emerged last night.
In an unprecedented development, the former Conservative leader will appear at the Supreme Court on Thursday in a case which will decide whether Mr Johnson acted illegally by proroguing Parliament earlier this month.
Sir John will be a lynchpin in the case brought by pro-Remain businesswoman Gina Miller, who said the Prime Minister may have breached 'the legal principle of Parliamentary sovereignty' and that the act represented an 'abuse of power'.
Yesterday, another former Tory prime minister, David Cameron, accused Mr Johnson of 'sharp practice' for proroguing Parliament.
But Mr Johnson said it was 'claptrap' for people to say he had stopped MPs having their say on Brexit.
He told the BBC: 'All this mumbo jumbo about how Parliament is being deprived of the opportunity to scrutinise Brexit.
'What a load of claptrap. Actually, Parliament I think has lost about four or five days.'
But those who brought legal challenges against the Prime Minister's decision argue the prorogation is designed to prevent parliamentary scrutiny of the UK's impending exit from the EU on October 31.
The Supreme Court, which will sit as a panel of 11 justices for only the second time in its 10-year history, must reconcile contradictory judgments issued by the English and Scottish courts.
The High Court in London dismissed the case brought by businesswoman and campaigner Gina Miller - who previously brought a successful legal challenge against the Government over the triggering of the Article 50 process to start the Brexit countdown - finding that the length of the prorogation was 'purely political'.
Giving reasons for their ruling on September 11, three of the most senior judges in England and Wales said: 'We concluded that the decision of the Prime Minister was not justiciable (capable of challenge). It is not a matter for the courts.'
But, on the same day, the Inner House of the Court of Session in Edinburgh ruled that Mr Johnson's decision was unlawful because 'it was motivated by the improper purpose of stymying Parliament'.
Lord Carloway, Scotland's most senior judge, said: 'The circumstances demonstrate that the true reason for the prorogation is to reduce the time available for parliamentary scrutiny of Brexit at a time when such scrutiny would appear to be a matter of considerable importance, given the issues at stake.'
Following that ruling, a Downing Street source suggested the MPs and peers who brought the legal challenge 'chose the Scottish courts for a reason' - prompting criticism from Scottish First Minister Nicola Sturgeon, who branded the comments 'deeply dangerous', as well as Justice Secretary Robert Buckland.
The Supreme Court, which will sit as a panel of 11 justices for only the second time in its 10-year history, must reconcile contradictory judgments issued by the English and Scottish courts
Mrs Miller's challenge was supported by former prime minister Sir John Major, shadow attorney general Baroness Chakrabarti and the Scottish and Welsh governments, who are all interveners in the Supreme Court case.
Boris Johnson's decision to prorogue Parliament for five weeks at a key pre-Brexit time for the country followed a well-trodden constitutional path.
The decision to shut down the legislature is ultimately taken by the Queen, but on the advice of the prime minister of the day and the Privy Council.
The monarch spoke with Mr Johnson by telephone before Commons Leader Jacob Rees-Mogg flew to Balmoral at the end of August to present the Government's plan in person.
She gave the Government a short window in which to carry out the prorogation and the decision was taken to enact it on Monday, after Boris Johnson made one last (failed) attempt to convince MPs to back his plea for a general election in October.
The decision of the Supreme Court in London on Tuesday would carry full political and legal weight.
As the court of last resort, if it upholds the ruling that Mr Johnson's advice to the Queen was unlawful it would effectively declare the shutdown null and void.
It is unclear exactly what would happen next - as Parliament would resume sitting, but the Government has not tabled any business.
A cross-party group of around 75 MPs and peers, led by SNP MP Joanna Cherry QC, was responsible for the Scottish challenge and the appeal against the Court of Session's decision is being brought by the Advocate General for Scotland, on behalf of the Westminster Government.
Victims' campaigner Raymond McCord - who brought separate proceedings in Belfast, arguing that a no-deal Brexit would damage the Northern Ireland peace process - has also been given permission to intervene in the Supreme Court case.
In a statement ahead of the hearing, Mrs Miller said: 'As with my first case, my Supreme Court case is about pushing back against what is clearly a dramatic overreach of executive power.
'This is an issue that cuts across the political divides - and the arguments about the EU - and it has united remainers and leavers and people of all political complexions and none in opposition to it.
'The precedent Mr Johnson will set - if this is allowed to stand - is terrifying: any prime minister trying to push through a policy that is unpopular in the House and in the country at large would from now on simply be able to resort to prorogation.
'No one could ever have envisaged it being used in this way: this is a classic power-grab.'
She added: 'The reason given for the prorogation was patently untrue and, since then, the refusal to come clean or provide any of the disclosures we have asked for has compounded the deception.
'It is my view - and the view of a great many others - that Mr Johnson has gone too far and put our parliamentary sovereignty and democracy in grave danger by his actions.'
SNP MP Joanna Cherry, pictured centre in