Boris Johnson’s suspension of Parliament was motivated by “political interest”, lawyers for ex-PM Sir John Major will argue at the Supreme Court.
The hearing on whether Mr Johnson’s decision to suspend Parliament was lawful is entering its final day.
The government has argued prorogation is not a matter for the courts, but critics accuse the PM of trying to stop MPs scrutinising his Brexit policy.
Parliament was prorogued earlier this month for five weeks.
In July, Sir John said he would be prepared to seek a judicial review if the new PM attempted to suspend Parliament.
He later joined the case brought by campaigner Gina Miller to avoid “taking up the court’s time”.
The court will hear his submission at 12:10 BST.
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What is happening today?
The panel of 11 justices began hearing submissions on behalf of the Scottish government at 10:30, followed by lawyers for the Northern Ireland victims’ campaigner Raymond McCord and then the Welsh government.
The Scottish government’s Lord Advocate James Wolffe QC said prorogation was taking place at a “time-critical period” when the government’s decisions would be “monumental”.
He said any decision to prorogue had to have “cogent justification”.
In a written submission, the Scottish government said prorogation would have a “profoundly intrusive effect” on Parliament.
Ronan Lavery QC, representing Mr McCord, urged the court to look at the impact prorogation would have on Northern Ireland. He said it was designed to “run down the clock” towards a no-deal Brexit which would lead to “the erection of the border” on the island of Ireland.
Supreme Court president Lady Hale said the court was “solely concerned” with the lawfulnesss of the prime minister’s decision to prorogue Parliament, not arguments about the nature of Brexit.
Another justice, Lord Wilson, later warned Mr Lavery not to “abuse our politeness or Lady Hale’s patience” by making arguments about Brexit.
In a written submission, shadow attorney general Baroness Chakrabarti said if the government’s power to prorogue Parliament was left “unchecked” MPs would be “deprived” of the ability to “perform their constitutional function”.
The government has also made a written submission setting out what it would do if the court found against Mr Johnson’s decision to suspend Parliament.
What is the court considering?
The Supreme Court is hearing two appeals.
The first is led by businesswoman Ms Miller who is appealing against the English High Court’s decision to throw out a challenge to prorogation.
The judges said the decision to suspend Parliament was “not a matter” for the judiciary.
The second comes from the government who are appealing against Scotland’s Court of Session ruling that prorogation was “unlawful” and used to “stymie” Parliament.
This challenge against the government was brought by 75 parliamentarians including the SNP MP Joanna Cherry.
What has happened so far?
Arguing on behalf of Ms Miller on the first day, Lord Pannick QC said there was “strong evidence” the PM wanted to “silence” Parliament.
However government lawyer Lord Keen QC argued that previous governments had prorogued Parliament to “pursue a particular political objective” and they were “entitled to do so”.
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On the second day the court heard from government lawyer Sir James Eadie QC who said the issue was not a matter for the courts.
He also argued that suspension had not silenced MPs because they had already managed to pass a bill blocking a no-deal Brexit, despite the prorogation dates.
In the afternoon session, Aidan O’Neil QC said the decision had been carried out “in bad faith”, and “for an improper purpose”.
What happens next?
It is not known when the judges will deliver their verdict, but it could be as early as Thursday afternoon.
Our home affairs correspondent said the feeling was the wait would not be long given the importance of the issue.
One senior government source told the BBC’s political editor No 10 believed the Supreme Court would judge that prorogation was a matter for the courts and would “fire warning shots about how a government should not use this to close Parliament illegitimately”.
However, Laura Kuenssberg said according to the source, No 10 did not believe the court would unravel their plan for a Queen’s Speech next month.
Supreme Court: Ex-PM’s lawyers to argue against prorogation}