Speaker John Bercow has declared that the House of Commons must re-convene "without delay" in the wake of the bombshell judgement of the Supreme Court that Boris Johnson's suspension of sittings was unlawful.
But there was no immediate guidance on how quickly MPs could be called back to Westminster.
The ruling came as prime minister Boris Johnson was in New York to address the United Nations General Assembly, while Labour MPs are in Brighton for their annual conference, with leader Jeremy Corbyn's crucial keynote speech due to take place at lunchtime on Wednesday. It was not immediately clear whether Mr Johnson will cut short his trip, due to end on Wednesday, to return to London.
Mr Bercow is seeking urgent talks with political party leaders to determine the way ahead.
In a statement, he said: "I welcome the Supreme Court’s judgement that the prorogation of Parliament was unlawful.
"The judges have rejected the government’s claim that closing down parliament for five weeks was merely standard practice to allow for a new Queen’s Speech.
"In reaching their conclusion, they have vindicated the right and duty of parliament to meet at this crucial time to scrutinise 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."
Any return to Westminster would throw into doubt the four-day Conservative annual conference, which is due to open in Manchester on Sunday and may have to be cut short.
Prior to the ruling, ministers did not rule out the possibility that Boris Johnson could request a fresh prorogration in order to suspend parliament once more
The House of Commons authorities said in a statement: “The House service's primary responsibility is to ensure the smooth running of parliamentary business. We are considering the implications of the Supreme Court’s judgment for parliament and will provide further information as soon as we can.”
Baroness Smith of Basildon, Labour's leader in the Lords, added that talks were under way about the resumption of parliament and she wanted the two Houses to be sitting from Wednesday.
Lady Smith labelled the ruling "phenomenally strong", adding: "For all 11 judges to come a unanimous decision, such a strong unanimous decision, I think it's a momentous decision for this country to say no elected politician, whatever their views, can override the rule of law."
Labour and the Liberal Democrats refused to take part in the prorogation ceremony in the House of Lords, with Lady Smith explaining: "We just felt it was wrong, we knew it was wrong and that decision has been vindicated."
On parliament's return, she added: "The judgment says get back as soon as possible. "I'd like to see us back tomorrow and I'm waiting for a phone call as we speak."
