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”.
What will happen today?
The panel of 11 justices will also hear submissions on behalf of the Welsh and Scottish governments and Northern Ireland victims’ campaigner Raymond McCord.
Lawyers for Mr McCord will argue the effects of the suspension of Parliament “have already been and will be more acute and severe” for Northern Ireland.
The Scottish government is expected to say prorogation will have a “profoundly intrusive effect” on Parliament, while lawyers representing the Welsh government will accuse Mr Johnson of “impeding” parliamentary sovereignty.
Shadow attorney general Baroness Chakrabarti will also make a written submission.
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”.
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.