The UK’s highest court declared that Boris Johnson went beyond his powers when he advised the Queen to suspend Parliament for five weeks.

A panel of 11 justices at the Supreme Court in London ruled unanimously on Tuesday that the Prime Minister’s advice to the Queen to prorogue Parliament until October 14 was unlawful because it had the effect of frustrating Parliament.

The court also held that the prorogation itself was “void and of no effect” and therefore Parliament had not been suspended.

Announcing the historic decision, the court’s president Lady Hale said: “That advice (to prorogue Parliament) was unlawful. It was outside the powers of the Prime Minister to give it.

“This means that it was null and of no effect.”

The judge said the prolonged suspension of parliamentary democracy took place in the “quite exceptional circumstances” of the UK’s impending exit from the EU on October 31.

She added: “Parliament, and in particular the House of Commons as the elected representatives of the people, has a right to a voice in how that change comes about.

“The effect upon the fundamentals of our democracy was extreme.”

The court concluded there was “no need” to consider Mr Johnson’s motive for the suspension.

The Prime Minister advised the Queen on August 28 to prorogue Parliament for five weeks, and it was formally suspended on September 9.

Mr Johnson claimed the five-week suspension was to allow the Government to set out a new legislative agenda in a Queen’s Speech when MPs return to Parliament.

But his legal opponents argued the prorogation was designed to prevent parliamentary scrutiny of the UK’s exit from the EU.

Mr Johnson was in the United States when the Supreme Court announced its findings, following an unprecedented hearing last week.

Over three days, the court heard appeals arising out of separate legal challenges in England and Scotland – in which leading judges reached different conclusions.

At the High Court in London, Lord Chief Justice Lord Burnett and two other judges rejected campaigner and businesswoman Gina Miller’s challenge, finding that the prorogation was “purely political” and not a matter for the courts.

But in Scotland, a cross-party group of MPs and peers led by SNP MP Joanna Cherry QC won a ruling from the Inner House of the Court of Session that Mr Johnson’s prorogation decision was unlawful because it was “motivated by the improper purpose of stymieing Parliament”.

Mrs Miller challenged the decision of the High Court, asking the Supreme Court to find that the judges who heard her judicial review action “erred in law” in the findings they reached.

During last week’s hearing, Lord Pannick QC, for Mrs Miller, told the packed court that Mr Johnson’s motive for a five-week suspension was to “silence” Parliament, and that his decision was an “unlawful abuse of power”.

He argued that Mr Johnson’s reasons for advising on a suspension of that length “were improper in that they were infected with factors inconsistent with the concept of parliamentary sovereignty”.

But Sir James Eadie QC argued on the Prime Minister’s behalf that the suggestion the prorogation was intended to “stymie” Parliament ahead of Brexit was “untenable”.

Mrs Miller’s case was supported by former prime minister Sir John Major, shadow attorney general Baroness Chakrabarti, the Scottish and Welsh governments, and Northern Irish victims’ campaigner Raymond McCord.

The justices were also asked by the Westminster Government to allow an appeal against the decision in Scotland.

The court allowed the appeal in Mrs Miller’s case and dismissed the appeal in the Scottish case.

Lady Hale said it was for Parliament to decide which steps to take following the ruling.

She said: “Unless there is some parliamentary rule of which we are unaware, they can take immediate steps to enable each House to meet as soon as possible.

“It is not clear to us that any step is needed from the Prime Minister, but if it is, the court is pleased that his counsel have told the court that he will take all necessary steps to comply with the terms of any declaration made by this court.”

At the outset of the judgment, given by Lady Hale and the court’s deputy president Lord Reed on behalf of all 11 justices, the judges said: “It is important to emphasise that the issue in these appeals is not when and on what terms the United Kingdom is to leave the European Union.

“The issue is whether the advice given by the Prime Minister to Her Majesty the Queen … was lawful.”

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