Former Tory prime minister Sir John Major will challenge Boris Johnson's decision to suspend parliament at the UK's highest court.
Sir John is among a host of supporters of a legal challenge in Scotland which found the prime minister's decision to prorogue - suspend - parliament was unlawful because it prevents MPs from scrutinising Brexit plans.
The ruling is being appealed by the government at the Supreme Court in London where Sir John's lawyer on Thursday - the third and final day - will put forward his argument that the suspension was motivated by Mr Johnson's "political interest" in closing down parliament ahead of the planned 31 October exit date.
A second ruling by the High Court, which rejected a challenge against Mr Johnson's prorogation move by anti-Brexit campaigner Gina Miller, is being appealed by her as part of the same hearing.
As well as Sir John, the panel of 11 justices on Thursday will hear submissions on behalf of the Welsh and Scottish governments and Northern Irish victims' campaigner Raymond McCord.
Lawyers for the Welsh government will say the prime minister's actions have "impeded" parliamentary sovereignty, while the Scottish government's legal team will insist the suspension 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, will tell the judges, through his lawyers, that the effects of the suspension "have already been and will be more acute and severe for the people of Northern Ireland" - where there has been no functioning government since March 2017.
On Wednesday, the government's lawyer argued the power to suspend parliament is a political matter and not for the courts to decide.
Sir James Eadie said if the Supreme Court intervened it would violate the "fundamental constitutional principle" of the separation of powers between courts and the government.
"This is, we submit, the territory of political judgment, not legal standards," he said.
On the second day of the hearings:
Mr Johnson announced in late August that he had asked the Queen to suspend parliament for five weeks until 14 October, when a new session would begin with a Queen's Speech and a fresh legislative agenda.
Critics who have taken their historic legal challenge to the UK's highest court of appeal claim the suspension was intended to prevent scrutiny of his belief that the UK should leave the EU by the deadline of 31 October, with or without a deal.
Mr Johnson has said the "prorogation", or suspension, was necessary to allow him to introduce a new legislative agenda.
If Sir James's argument is accepted, any investigation by the court into issues including if Mr Johnson lied or what his motives were would be dropped.
He made the case that even if there was a political advantage to suspending parliament, in the eyes of the law that does not mean it is unlawful or a matter for the courts.
His case against Gina Miller convinced High Court judges a few weeks ago.
The businesswoman's case wanted the judges to find that prorogation for an "exceptional" length of time was an "unlawful abuse of power", but the court ruled suspending parliament was "inherently political in nature and there are no legal standards against which to judge the legitimacy".
However, a separate case brought by SNP MP Joanna Cherry in Scotland was successful, with judges ruling the prorogation was unlawful.
The court found the suspension "an egregious case of a clear failure to comply with generally accepted standards of behaviour of public authorities".
Speaking in court today, Sir James said in his opening remarks: "We are dealing with a prerogative power.
"It is a prerogative power that has been expressly preserved by parliament."
One justice posed the question of what would happen if a PM decided he did want to stifle the scrutiny of his actions and prorogued for a year. Sir James acknowledged the potential extreme use of it but told the court that principles shouldn't be established by being tested against the extreme.
He said there are controls that can be set against extreme circumstances and that parliament would exercise control over the government in all that.
Sir James also argued that MPs had their chance to legislate against prorogation when they passed a bill that forces the government to seek a Brexit extension in case of no-deal, saying the MPs could have used that piece of legislation also to avoid the suspension.
Pointing the judges to the cabinet minutes and memos to Mr Johnson which have been submitted as evidence, Sir James said it was clear that the main aim of the prorogation was to get to a position in which a new Queen's Speech could be prepared and not for any "improper purpose".
However, Aidan O'Neill QC, representing a group of around 75 MPs and peers led by Ms Cherry, told the justices that "closing down parliament is a derogation from that general constitutional principle of accountability" and was "unlawful because it had the purpose of stymieing parliament".
He argued the length of suspension is not important, rather the intent and effect of it is.
Mr O'Neill also urged the justices not to treat the government's published documents as "gospel" because it can't be certain they are "the complete picture."
"Once parliament has been prorogued, the only constitutional actor left standing is the courts," he said.