The UK Supreme Court on Tuesday ruled that Prime Minister Boris Johnson's suspension of Parliament was illegal.

Judges in the country's highest court unanimously decided that the suspension of Parliament was "void," meaning members of Parliament must return immediately.

The verdict led to immediate calls for Johnson to resign and to apologize for misleading the Queen when he requested the prorogation of Parliament.

Johnson said the court was putting his Brexit plans at risk.

The House of Commons speaker, John Bercow, said Parliament would return Wednesday.

Johnson was expected to fly back early from New York on Tuesday after addressing the UN General Assembly.

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The UK Supreme Court has unanimously ruled that Prime Minister Boris Johnson's suspension of Parliament was illegal and had "no justification," a dramatic ruling that immediately triggered calls for him to resign.

The country's highest court ruled Tuesday that Johnson's decision was "unlawful" and that the prorogation of Parliament was "void," meaning members of Parliament will return to Westminster this week.

Johnson said he "strongly disagrees" with the verdict but accepted that Parliament must return.

The prime minister closed down Parliament for five weeks earlier this month in a move opponents argued was designed to stifle their efforts to legislate against a no-deal Brexit in October.

Read more: Boris Johnson told to resign following Supreme Court defeat and become the 'shortest-serving prime minister ever'

Read more: What happens now that the Supreme Court has ruled Boris Johnson broke the law by suspending Parliament?

Read more: Read the UK Supreme Court's explosive verdict on why Boris Johnson's suspension of Parliament was illegal

He had insisted his suspension was designed purely to allow time to put together a new program for his premiership and argued the court should not intervene in a political matter.

The court's 11 judges upheld the earlier verdict of the Scottish Court of Session, however, ruling that Johnson's decision to close down Parliament was a deliberate attempt to "stymie" the role of lawmakers.

"This was not a normal prorogation in the run-up to a Queen's Speech," Lady Hale, the Supreme Court president, said as she delivered the verdict Tuesday morning. "The effect on the fundamentals of our democracy was extreme. No justification for taking action with such an extreme effect has been put before the court."

She added: "This court has already concluded that the prime minister's advice to her majesty was unlawful, void, and of no effect.

"This means that the Order in Council to which it led was also unlawful, void, and of no effect and should be quashed. This means that when the royal commissioners walked into the House of Lords it was as if they walked in with a blank sheet of paper.

"The prorogation was also void and of no effect. Parliament has not been prorogued."

"This is the unanimous judgment of all 11 justices."

Johnson lashes out at Supreme Court

Speaking with reporters in New York, where he was attending the UN General Assembly, Johnson accused his opponents of trying to "frustrate Brexit" and accused the court of wrecking his hopes of negotiating a new exit deal with the European Union.

"I strongly disagree with this decision," Johnson said.

"I have the upmost respect for our judiciary — I don't think this was the right decision. I think that the prorogation has been used for centuries without this kind of challenge."

He added: "Let's be in no doubt there are a lot of people who want to frustrate Brexit. There are a lot of people who want to stop this country coming out of the EU."

Read more: 'There are a lot of people who want to frustrate Brexit': Boris Johnson accuses Supreme Court of sinking deal with the EU

A representative for the prime minister said he would not be resigning but would return home early in time to be back in Parliament on Wednesday.

What happened?

Demonstrators outside the Supreme Court in London on Tuesday. Reuters / Henry Nicholls

The Supreme Court's 11 judges dealt with two appeals relating to Johnson's decision over a three-day hearing.

The first was the UK government's appeal against Scotland's Court of Session, ruling that Johnson's move to suspend Parliament was unlawful and intended to stifle parliamentary scrutiny of his handling of Brexit.

The second was from Gina Miller, the activist who took an appeal to the Supreme Court after the English High Court ruled that Johnson's proroguing of Parliament was entirely political and not a matter for judges.

The UK government previously insisted it would abide by the verdict of the court, meaning Parliament could be reconvened immediately.

'Momentous'

The activist Gina Miller. Leon Neal/Getty Images

Following the verdict, Jeremy Corbyn, the leader of the opposition Labour Party, called on Johnson to "consider his position" and "become the shortest-serving prime minister there's ever been."

Scottish National Party MP Joanna Cherry, who helped bring the case in Edinburgh's Court of Session, said Johnson's position had become "untenable" and called on Parliament to resume "immediately the important job of scrutinizing this minority Tory government."

She said: "This is a huge victory for the rule of law and for democracy and is very much in keeping with the Scottish constitutional decision that neither the government, or indeed the monarch, are above the law."

She added: "His position is untenable and he should have the guts for once to the decent thing and resign."

The House of Commons speaker, John Bercow, called on Parliament to reconvene immediately.

"I welcome the Supreme Court's judgment that the prorogation of Parliament was unlawful," he said in a statement.

"In reaching their conclusion, they have vindicated the right and duty of Parliament to meet at this crucial time to scrutinize the executive and hold ministers to account. As the embodiment of our parliamentary democracy, the House of Commons must convene without delay. To this end, I will now consult the party leaders as a matter of urgency."

Parliament is due to return at 11:30 on Wednesday morning.

This means members of Parliament, who are taking part in a series of party conferences, may have to return to London as early as Wednesday.

Corbyn, who was at the party's annual conference in Brighton, brought forward a speech scheduled on Wednesday to Tuesday afternoon to facilitate that plan.