A defiant Boris Johnson has vowed to push ahead with Brexit despite a landmark Supreme Court ruling that his decision to suspend parliament was “unlawful”.

Amid mounting calls for his resignation, the Prime Minister said he disagreed with the 11-0 judgment in Britain’s highest court, described by some as “a legal earthquake”.

The political ramifications of the verdict are huge, with Mr Johnson likely to face a vote of no-confidence when Parliament returns, a general election likely to be called, and the possibility of a second Brexit referendum.

However, the unwavering Prime Minister appears determined to press on.

“As the law currently stands, the UK leaves the EU on October 31st come what may. But the exciting thing for us now is to get a good deal,” Mr Johnson said in New York on Tuesday, where he was attending the United Nations climate summit.

“That is what we are working on. And to be honest it is not made much easier by this kind of stuff in parliament or in the courts,” he said.

“I am very hopeful that we will get a deal and I think what the people of the country want is to see parliamentarians coming together working in the national interest to get this thing done and that is what we are going to do.”

A Downing Street source said Mr Johnson would not resign and would fly back to London a day earlier, after his speech to the UN on Tuesday.

Mr Johnson had advised the Queen to prorogue, or suspend, the British parliament for five weeks until mid-October, which sparked uproar.

Announcing the verdict, Supreme Court president Lady Brenda Hale said the suspension was “justiciable” – capable of challenge in the courts – and that the case was ‘‘about the limits on the power to advise the Queen to prorogue parliament’’.

Recognising “this was not a normal prorogation”, Lady Hale said it “prevented parliament from carrying out its duties” in the crucial period before the October 31 Brexit deadline.

“The court is bound to conclude that the decision to advise the Queen to prorogue parliament was unlawful,” Lady Hale said.

Outlining the justices’ view that Mr Johnson’s proroguing was ‘‘unlawful, void and of no effect and should be quashed’’, Lady Hale said it was up to the Speaker to decide what happens next.

Here, read up on Brenda Hale, President of the U.K. Supreme Court, who just snatched Boris Johnson's wighttps://t.co/nSkj426Xuf

— boygobong (@boygobong) September 24, 2019

The outgoing Speaker of the House of Commons, John Bercow, said he welcomed the court’s judgment.

“As the embodiment of our parliamentary democracy, the House of Commons must convene without delay,” Mr Bercow said.

“To this end, I will now consult the party leaders as a matter of urgency.”

Parliament has since been recalled for an 11.30am start on Wednesday (British time).

Mr Johnson said he would “respect” the court’s decision.

“Obviously this is a verdict that we will respect and we respect the judicial process,” Mr Johnson said.

“I have to say that I strongly disagree with what the justices have found.

“I don’t think that it’s right, but we will go ahead and, of course, parliament will come back.”

Shortly after the verdict, British Labour leader Jeremy Corbyn called on Mr Johnson to consider his position and call a new election.

“I invite Boris Johnson, in the historic words, to ‘consider his position’,” Mr Corbyn told delegates at the Labour Party’s annual conference in Brighton.

To huge cheers and chants of “Johnson out!”, Mr Corbyn said the PM should become Britain’s shortest-serving leader and that Labour was ready to form a government.

Labour leader Jeremy Corbyn says UK PM Boris Johnson should "consider his position", after the Supreme Court found his suspension of Parliament was unlawful

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— BBC Politics (@BBCPolitics) September 24, 2019

Former PM John Major was also scathing in his criticism.

“Parliament must now be recalled immediately to recommence its work, and to receive the Prime Minister’s unreserved apology,” Mr Major said.

“… No prime minister must ever treat the monarch or parliament in this way again.”

-with AAP