Boris Johnson was called the "father of lies" at the Supreme Court during a hearing over his decision to prorogue Parliament.
A panel of 11 justices was urged to “stand up for democracy” when reaching their decision over the Prime Minister’s controversial decision to suspend Parliament from September 9 to October 14.
They were hearing appeals arising from separate challenges in the English and Scottish courts, in which leading judges reached differing conclusions after Mr Johnson’s motives were called into question.
Aidan O’Neill QC, representing a group of around 75 MPs and peers led by SNP MP Joanna Cherry QC, urged the justices to dismiss the government’s appeal against a ruling of the Inner House of the Court of Session in Edinburgh, which concluded the prorogation was “unlawful”.
Mr O’Neill told the justices: "Stand up for truth, stand up for reason, stand up for diversity, stand up for Parliament, stand up for democracy by dismissing this Government appeal and upholding a constitution governed by laws, not the passing whims of men.
"What we have with prorogation is the mother of parliaments closed down by the father of lies."
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The Prime Minister claimed he wanted Parliament to shut so he could hold a standard Queen’s speech but Mr O'Neill said Mr Johnson couldn’t be trusted.
He described the PM as being capable of using “low, dishonest, dirty tricks” and said he was also concerned a witness statement had not bee supplied, adding: “An affidavit would say this is the truth, the whole truth and nothing but the truth."
Mr O'Neill labelled the government as “unworthy of our trust” and cast doubt on the honesty of documents it had submitted in the case as evidence.
He ended his address by saying the “court should listen to the angels of its better nature and rule that this prorogation is an unlawful abuse of power of prorogation.”
Scottish courts ruled on September 11 Mr Johnson’s advice to the Queen to prorogue Parliament for five weeks was unlawful because it was “motivated by the improper purpose of stymieing Parliament”.
At the outset of his arguments, Mr O’Neill said the Inner House had the advantage of “distance” over the High Court in London – which concluded the prorogation was not a matter for the courts in a case brought by campaigner and businesswoman Gina Miller.
Sir James Eadie QC, representing the Prime Minister, argued the justices did not have jurisdiction to determine if the prorogation was unlawful.
Submissions will be made to the Supreme Court until Thursday and a ruling will be made after this.