The ruling is a blow to Johnson, who moved to suspend Parliament last month as part of his larger effort to wrest the UK from the European Union with or without a negotiated deal.
Johnson's tough stance has drawn sharp criticism from other members of Parliament, including members of his own party.
The Supreme Court said the decision to shut down Parliament was unlawful and unconstitutional. It upholds the ruling of Scotland’s Supreme Court.
Soon after the ruling was announced to gasps in the courtroom on Tuesday, MEPs were quick to react, with SNP deputy Alyn Smith demanding the UK Parliament resume its work “immediately.”
Smith said, “This is good news, and I pay tribute to my SNP colleague Joanna Cherry QC MP who led the case.”
“The UK Parliament must resume without delay to hold the Tory government to account on its Brexit plans, which threaten to plunge the UK into recession, destroy 100,000 Scottish jobs, and inflict lasting harm on living standards, public services and the economy across Scotland, the UK and the EU.”
“Boris Johnson, already a laughing stock in Brussels, should resign. His behaviour has been disgraceful and his position is untenable.”
“Boris Johnson, already a laughing stock in Brussels, should resign. His behaviour has been disgraceful and his position is untenable” Alyn Smith MEP
Guy Verhofstadt, the Belgian MEP who heads Parliament’s Brexit Steering Group, said the decision to rule Johnson's suspension of parliament as unlawful was a "big relief in the Brexit saga.”
“The rule of law in the UK is alive and kicking. Parliaments should never be silenced in a real democracy," said the former Prime Minister.
"I never want to hear Boris Johnson or any other Brexiteer say again that the European Union is undemocratic,” he added.
Fellow Belgian MEP Philippe Lamberts, the co-leader of the Greens in Parliament and a fierce critic of Johnson, said: “The decision confirms what we already knew about Johnson. That he will spare no tricks to get what he wants. He is an ample opportunist. But I look at Jeremy Corbyn and I see a lot of opportunism there as well.”
A Commission spokesman told reporters, “We've all seen the news of course, but it is not for us to comment on the internal constitutional matters of Member States, and this includes the UK.”
“The rule of law in the UK is alive and kicking. Parliaments should never be silenced in a real democracy" Guy Verhofstadt MEP
The co-chairs of the European Green Party Monica Frassoni and Reinhard Bütikofer said in a statement, “Today’s landmark ruling by the Supreme Court in the UK that the suspension of Parliament was unlawful marks a monumental victory for the sovereignty of parliamentary democracy. It sends out a clear message that no-one is above the law.”
“We have been steadfast in our support for our Green colleagues in the UK and totally against the shutting down of Parliament to override parliamentary scrutiny. The rule of law is one of the fundamental values of the European Union and its Member States and cannot be discarded on a whim.”
“Elected representatives in the UK must now be allowed to resume work as a matter of urgency so they can begin to chart a sensible path through this national crisis. We believe the final decision on the UK's future in the European Union should be made by the people through a referendum - a Peoples Vote.”
“The decision confirms what we already knew about Johnson. That he will spare no tricks to get what he wants. He is an ample opportunist” Philippe Lamberts MEP
Elsewhere, Brendan Donnelly, former Conservative MEP and director of the Federal Trust think tank in London, said, "The court has taken a massive step towards defending democracy in the UK. Parliament should take another by voting No Confidence in the discredited Johnson government."
Roger Casale, a former Labour MP and founder of the civil rights group New Europeans said, "By shutting down Parliament and refusing to confirm he will obey the rule of law, Boris Johnson has shown that he is willing to wreak havoc with British democracy.”