Theresa May should resign after losing any vote of no confidence in the Commons and recommend a successor, a committee of MPs has concluded.
Its report clears up confusion about the power of MPs to topple the prime minister by finding that any no confidence vote – even on a non-binding motion – removes the authority to govern.
Ms May would be expected to resign “unless that authority could be restored” in a second vote and recommend a successor to the Queen.
The conclusion comes as Jeremy Corbyn faces growing pressure to push for a no-confidence vote from the leaders of all other opposition parties at Westminster, because of the deepening Brexit chaos.
The inquiry was launched by the Public Administration and Constitutional Affairs Committee because of “unanswered questions”, following the passing of the 2011 Fixed-term Parliaments Act.
That Act provides no guidance on what should happen if a no-confidence motion is passed and attempts are being made to form a new government, without a general election.
But Bernard Jenkin, the committee’s chairman, said: “It is fundamental to our democratic system that the government commands the confidence of the elected House of Commons.
“We have made clear, for both MPs and the public, what would be expected to happen if the House were to express ‘no confidence’ in the government.”
Should the Conservatives be unable to choose a new prime minister, an election could then be held under laws of the 2011 Act.
The Act means that early polls – such as Ms May’s disastrous 2017 snap election – can only be triggered immediately if two-thirds of MPs vote for it.
If a vote of no-confidence is lost on a straight majority, the government has 14 days to reverse that, with a general election following if it fails to do so.
Crucially, in its report, the committee says that provision “in no way affects the fundamental principle that the government’s authority to govern rests on the confidence of the House, however it chooses to express it”.
It added: “Unless that authority could be restored, the prime minister would be expected to give notice that he or she will resign, but only when he or she is in a position to recommend to the sovereign an alternative person to form a new administration.
“In the event that no alternative person can be found, it remains available to the House to bring about an early general election.”
The report came as the Liberal Democrats, the SNP, Plaid Cymru and the Greens joined forces to demand Labour table a motion of no confidence in the government this week.
Labour has said it will only do that “when it was most likely to be successful”, suggesting it will wait until after the prime minister has returned to the Commons with a revised deal.
But, the joint letter says: “The government’s inability to pass its Brexit deal through parliament, as witnessed by Theresa May’s withdrawal of her own motion in parliament yesterday, leaves no option for us as leaders of opposition parties but to present a confidence motion.
“It is our intention to table a motion in the names of the Westminster parliamentary leaders of the Scottish National party, the Liberal Democrat party, Plaid Cymru (The Party of Wales) and the Green party. We hope you will join us.”