As the Supreme Court gets ready to hear from a host of supporters of a legal challenge over the controversial prorogation of Parliament, eyes will be firmly fixed on what the former Prime Minister and Tory leader will have to say.
Sir John, who prorogued Parliament himself for three weeks in 1997, will argue that Mr Johnson had “ulterior motives” when he suspended Parliament at the start of September.
Citing a previous legal case where an estate agent was found to be in “breach of fiduciary duty” for claiming that the buyer of a property wanted to live in it but really wanted to sell it on for profit, Sir John will make the comparison with the PM.
In submissions drawn up by his lawyers, Sir John has said: “It could hardly be suggested that the duties of the Prime Minister to the monarch are less than those of an estate agent to a homeowner.
“Accordingly, if the court is satisfied that the Prime Minister’s decision was materially influenced by something other than the stated justification, that decision must be unlawful.”
Sir John will argue that Mr Johnson’s reasons for proroguing Parliament - to bring forward a Queen’s Speech - “make no sense and cannot be the true explanation”.
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The former PM will not address the court himself, but his lawyers will put forward his argument that the suspension was motivated by Mr Johnson's "political interest" in closing down Parliament ahead of the UK's planned exit from the European Union on October 31.
Lawyers for businesswoman and campaigner Gina Miller argue Mr 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 the historic hearing, 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.
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.
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”.
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.
It is not known when the court is expected to give its ruling.
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