A Scottish court's decision that the prorogation of Parliament is unlawful means MPs should be recalled, Welsh opponents of Boris Johnson have said.
The judges found in favour of a cross-party group of politicians, including a number of Welsh Labour and Plaid MPs.
Plaid's Hywel Williams said the UK was "hurtling headlong" into a crisis, which MPs must be sitting in the House of Commons to stop.
The Supreme Court is to consider UK ministers' appeal against the ruling.
That will take place in London next week. The suspension of Parliament began in dramatic scenes early on Tuesday morning.
Ministers in the UK government have said the prorogation is normal procedure ahead of a Queen's Speech - when ministers spell out their legislative plans - and a recess was likely to take place in any case.
But its implementation before Brexit triggered protests across the UK.
Opponents say it has left them with less time to scrutinise the government ahead of exit day, 31 October, amid fears the UK could leave the EU without agreeing a deal in Parliament.
Plaid Cymru's Hywel Williams, one of the petitioners to the case, said: "Boris Johnson has now broken the law. He has held democracy in such contempt that judges have had to step in. Parliament should be recalled immediately."
"We are hurtling headlong towards a political, economic and constitutional crisis of proportions perhaps never seen before. Parliament must be sitting to stop it."
The petitioners to the Scottish Court of Session case included 13 Welsh MPs. All four Plaid MPs, Geraint Davies, Tonia Antoniazzi, Wayne David, Stephen Doughty, Ruth Jones, Anna McMorrin, Madeline Moon, Owen Smith and Jo Stevens took part, alongside Lord Hain.
Independent former Welsh Conservative MP Guto Bebb said Boris Johnson should resign if he has misled the Queen.
He told Radio 4's World at One: "If it transpires that by offering what is deemed to be unlawful advice to her Majesty the Queen, it transpires that the prime minister misled the Queen, then I think his position is untenable."
Kevin Brennan, Labour MP for Cardiff West, tweeted a picture of himself returning to the Commons, which is currently open to tourists while proceedings are suspended.
"I'm heading down too," replied Stephen Doughty. "Time we all were back instead of blocked from holding this reckless lying government to account."
'Trying to interfere'
But David Davies, Brexiteer Monmouth Conservative MP, said he did not believe such matters were "in the purview of the court".
"I think it's disappointing that the courts are trying to interfere in the way the country is run," he said.
Montgomeryshire Conservative Glyn Davies said: "I was surprised by today's judgement because I personally saw prorogation, coinciding with conferences recess, as sensible prior to a desperately needed Queen's Speech."
Welsh Liberal Democrat MP Jane Dodds said the proroguing of Parliament was "authoritarian". "Boris Johnson is trampling on the very values and principles the United Kingdom was founded upon," she said.
A spokesman for No 10 said it was "disappointed" by the decision, and would appeal to the Supreme Court.
He added: "The UK government needs to bring forward a strong domestic legislative agenda. Proroguing Parliament is the legal and necessary way of delivering this."
The UK government said Parliament remains prorogued pending the decision of the Supreme Court.
The Welsh Government in Cardiff intends to be a party to the Supreme Court case - it has opposed prorogation and supported Gina Miller's High Court challenge, which was rejected.
Welsh Government First Minister Mark Drakeford said: "We'll be represented in court in order to explain why proroguing Parliament, just at a point when we were relying on that parliament to discharge a whole raft of legislation that will be important here in Wales, was not in the interests of Wales and not carried out in line with the requirements of the law."