But Johnson lashed out in response, saying he did not think it was the “right decision”.
He said he had “utmost respect for our judiciary” but added “let’s be in no doubt there are a lot of people who want to frustrate Brexit”.
The pro-Brexit press concluded – in the words of the Daily Mail – Johnson had “declared war on the judiciary”, quoting a senior ally of the PM asking “who runs this country?”
Jacob Rees-Mogg, a cabinet minister and the Leader of the House of Commons, was widely reported to have described the court’s decision as a “constitutional coup” during a trans-Atlantic conference call between Johnson and the Ccabinet.
Another cabinet minister told the BBC justices had “made a serious mistake in extending its reach to these political matters” and a senior Conservative told them “this is now literally the people versus the establishment”.
A government source told London’s Telegraph: “The massed ranks of the establishment are trying to frustrate the biggest democratic vote in our history.”
They added: “Every time the courts and others try to stop the Prime Minister, it reinforces the point that the government is fighting a lone battle to carry out the will of the people.”
It follows another anonymous government figure warning in the Sunday Times that judges should “reflect deeply on the profound consequences for the judiciary if they are seen by the public to side with those trying to cancel the biggest democratic vote in our history”.
But the inflammatory language prompted alarm in legal quarters.
Chair of the Bar Council of England and Wales Richard Atkins said the country had “reached a low point in our history” when the government was issuing threats to the judiciary.
“The rule of law and the independence of the judiciary are fundamental pillars of our constitution and our democracy – as [the Supreme Court’s] judgment proves,” he said.
Lawyers associated with the Supreme Court action had received death threats with their family home address published on social media.
“In other countries and fragile states we have seen the consequences which follow when the rule of law has been disregarded if not openly abandoned,” Atkins said.
Former Supreme Court justice Lord Sumption told the BBC the country’s flexible constitution had responded “to attempts to abuse its flexibility”.
“That is not undermining democracy at all – nor is it a coup – it is simply replacing what ought to have happened by convention, by law in circumstances where the government has tried to kick away the conventions.”
One of Australia’s leading constitutional lawyers Professor Anne Twomey, whose work was consulted by the Supreme Court during its deliberations, said the court’s decision was not remarkable – but took a similar approach to that taken in many countries including Australia’s High Court.
“This was about the judiciary ensuring parliament can fully exercise its role and not be overcome by executive power inappropriately,” she said.
But she said the ruling sidestepped a few key questions – such as the role of the Queen – which may mean the issue is not closed yet, and could be decided another way if it came to court in Australia.
Nick Miller is Europe correspondent for The Sydney Morning Herald and The Age