BORIS Johnson has been branded the “father of lies” on the second day of the historic legal case which could ultimately decide his premiership.

The Prime Minister was also accused of being open to “low, dishonest, dirty tricks” by those challenging his decision to suspend parliament for five weeks.

A full panel of 11 justices at the UK Supreme Court is being asked to give a definitive ruling on whether Mr Johnson exceeded his powers by asking the Queen to prorogue parliament.

Businesswoman Gina Miller is appealing against a decision by the High Court in London last month, which ruled the issue was an entirely political matter, rather than one of law.

Meanwhile the UK Government is appealing against a decision of the Inner House of the Court of Session in Edinburgh last week, when three judges ruled the prorogation was unlawful because its goal was to “stymie” scrutiny of Brexit before the October 31 deadline.

It followed a group of 75 MPs and peer led by SNP MP Joanna Cherry QC challenging the suspension.

The implication of the Inner House’s ruling was that the PM had lied to the Queen when he asked for her consent on August 28 to prorogue parliament from September 9 to October 14. Former Tory Attorney General Dominic Grieve has said that would be a resignation issue.

Mr Johnson has said the prorogation, the longest outside an election since the 1930s, was to tee up new legislation.

However Aidan O’Neill QC, for the Scottish campaigners, told the Supreme Court it was motivated by an improper purpose and so “done in bad faith”.

He said redacted documents supplied by Number 10 - including a memo to the PM and his handwritten response - showed “the true dominant purpose of prorogation was, as the Inner House correctly observed, to stymie parliamentary scrutiny of the executive regarding Brexit”.

He added: “Lying (albeit wholly unconvincingly) about the true reasons for exercising the prorogation power in the manner, at the time and for the period it has been exercised in this case, calls into question the lawfulness of the executive’s action.”

Referring to the Government’s failure to submit a witness statement to the court, Mr O’Neill said it could normally be assumed that the Government would not resort to “low, dishonest, dirty tricks - but I’m not sure we can assume that of this Government”.

Submitting the Supreme Court did have the power to rule Mr Johnson’s decision unlawful, Mr O’Neill said: “Once Parliament has been prorogued, the only constitutional actor left standing is the courts.”

He added: “We cannot have a situation in which there are no standards, in which prorogation can be used with impunity.”

Closing, he cited the infamous Dred Scott case, when the US Supreme Court ruled in 1857 that black people could not be Amercian citizens even if free.

He said: “I say to this court, don’t let this case be your Dred Scott moment.

“Instead 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. Lies have consequences - but the truth will set us free.

“Rather than allow lies to triumph, this court should listen to the angels of its better nature and rule that this prorogation is an unlawful abuse of power of prorogation which has been entrusted to the Government.

“But this Government has shown itself unworthy of our trust as it uses the power of office to which is corrosive of the constitution and destructive of the system of parliamentary representative democracy on which our union polity is founded. Enough is enough. Dismiss this appeal and let them know that. This is what truth speaking to power sounds like.”

It was the end of a colourful, often florid, romp through history that covered 1066, Macbeth, James VI, Bannockburn and Braveheart, and appeared to draw mixed reviews from the justices.

Jolyon Maugham QC, of the Good Law Project, which supported the Scottish case, said it had been intended to focus on first principles and the scale of the issues at stake.

He said the “technocratic” arguments had been covered the previous day by Lord Pannick QC, who was acting for Ms Miller, and there was no point in Mr O’Neill repeating them.

Earlier, Sir James Eadie QC, for the UK government, said prorogation fell squarely within a class of executive decisions which were “inherently and fundamentally political in nature”.

He said the suggestion it was done to stymie parliament was “untenable”.

However he faced a series of questions about the practical effect of prorogation.

Lady Black said the Government was arguing the court could not get involved because Parliament already acted as a check on the Government, but how could Parliament apply a check on government once Parliament was “removed from the picture”?

Sir James said prorogation always and inevitably had the effect of limiting debate in Parliament, but MPs could resume their scrutiny once the suspension was over.

Sir James also admitted that the government might have derived some political advantage from choosing the length of prorogation it had, and that it could have been far shorter. He also said governments could use a “double prorogation” if it suited them.

Lord Wilson asked why no witness statement had been provided to explain why the prorogation decision was taken.

He said: “Isn’t it odd that nobody has signed a witness statement to say ‘this is true, these are the true reasons for what was done’?” Sir James replied: “We have the witness statements we have.

“My submission is that, in light of the case law only, it remains open to the court to make judgments on the facts on the basis of the underlying documents that have been produced.”

Mr O’Neill later warned the few documents which had been supplied by Number 10 should not be treated as “gospel” or “complete truth”.

Today’s final day of hearings is due to include potentially dramatic evidence from former Tory PM Sir John Major.