Sir John Major is today set to tell the UK’s highest court that Boris Johnson’s decision to prorogue Parliament was motivated by the Prime Minister’s “political interest”.
On the final day of the historic Supreme Court battle, the former-PM will make the case in written submissions that Mr Johnson’s shutdown was “unlawful”.
He will not himself address the court but his lawyers will argue that the PM acted to " deprive Parliament of a voice" by moving to suspend it for five weeks.
In his written submission, seen by The Times, Sir John will add: "The inference was inescapable that the prime minister’s decision was motivated, or in any event substantially motivated, by his political interest in ensuring that there was no activity in parliament during the period leading up to the EU Council summit on 17-18 October.”
The UK's highest court is set to hear from a host of other supporters of a legal challenge over the controversial prorogation of Parliament.
Lawyers for businesswoman and campaigner Gina Miller argue Prime Minister Boris Johnson's advice to the Queen to suspend Parliament for five weeks was "an unlawful abuse of power".
On Thursday, the third and final day of a historic hearing at the Supreme Court in London, a panel of 11 justices will hear submissions on behalf of Sir John, the Welsh and Scottish governments and Northern Irish victims' campaigner Raymond McCord.
Prime Minister’s motive for prorogation was to silence Parliament, court hears
Lawyers representing the Welsh Government will say Mr Johnson's actions have "impeded" parliamentary sovereignty, while Holyrood's legal team will submit the prorogation will have a "profoundly intrusive effect" on Parliament's ability to scrutinise the executive branch of the Government.
Mr McCord, whose son was murdered by loyalist paramilitaries in 1997, brought separate legal proceedings in Belfast arguing that a no-deal Brexit would damage the Northern Ireland peace process.
His lawyers will tell the Supreme Court judges that the effects of Parliament's suspension "have already been and will be more acute and severe for the people of Northern Ireland".
The court has also received written submissions from shadow attorney general Baroness Chakrabarti.
The justices are hearing appeals arising from earlier rulings in which leading judges reached different conclusions.
At the High Court in London, the Lord Chief Justice Lord Burnett and two other judges rejected a challenge against the Prime Minister's prorogation move by Mrs Miller.
But in Scotland, a cross-party group of MPs and peers won a ruling from the Inner House of the Court of Session that Mr Johnson's prorogation decision was unlawful because it was "motivated by the improper purpose of stymieing Parliament".
Supreme Court begins hearing legality of prorogation
Mrs Miller is now appealing against the decision of the High Court, asking the Supreme Court to find that the judges who heard her judicial review action "erred in law" in the findings they reached.
Her barrister, Lord Pannick QC, told the court on Tuesday that Mr Johnson's motive for an "exceptionally long" prorogation was to "silence" Parliament, and that his decision was an "unlawful abuse of power".
Sir James Eadie QC argued on the Prime Minister's behalf on Wednesday that the suggestion the prorogation was intended to "stymie" Parliament ahead of Brexit is "untenable".
The justices are also being asked by the Westminster Government to allow an appeal against the decision in Scotland.
The Prime Minister advised the Queen on August 28 to prorogue Parliament for five weeks and it was suspended on September 9.
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.
But those who brought the legal challenges argue the prorogation is designed to prevent parliamentary scrutiny of the UK's impending exit from the EU on October 31.
It is not known when the court is expected to give its ruling.
During a brief discussion on Wednesday about what order the court should make in the event it concludes the prorogation was unlawful, Lord Reed said the issue could be a "very difficult question" for the judges.