Opposition party leaders are meeting in Westminster to discuss how to ensure Boris Johnson cannot push a no-deal Brexit through against the will of parliament.
As part of the discussions, Jo Swinson is expected to propose an amendment to the Benn Act which would force the prime minister to seek an extension to Article 50 as early as this weekend.
The Benn Act already requires Johnson to take the request to Brussels if he does not have an acceptable deal by October 19, but opposition leaders fear that this does not leave enough time for legal challenges if Johnson tries to circumvent it.
The prime minister and government representatives have said they will follow the law but continue to insist the UK will leave the EU by October 31.
Yet there is no breakthrough in Withdrawal Agreement negotiations on the horizon.
A Lib Dem source told the Press Association that Swinson will put forward the idea of bringing forward the deadline for seeking an extension when she meets counterparts from Labour, the SNP, Plaid Cymru and the Greens.
“I expect it to be discussed at the meeting. It is certainly Jo’s intention that it should,” the source told PA.
Ian Blackford, the SNP leader at Westminster, signalled his support for the Lib Dem plan, saying he would back any measure which prevented Johnson bypassing the Benn Act.
“All of us have got to work together. I’m not precious. Everyone is much more aware of what he is capable of,” he told The Daily Telegraph.
“There is no doubt that he will seek to frustrate the legislation in place as we head towards the second half of October.”
A Labour source said: “We are looking at all mechanisms and additional legal safeguards to block no-deal and ensure the PM complies with the Benn Act.”
The move could see Tory MPs and ministers forced to abandon their party conference in Manchester and dash back to Westminster if the opposition stage an ambush to seize control of the Commons agenda.
Unusually, the Commons is sitting during conference after MPs refused to grant the government the customary recess.
The government’s own prorogation of parliament, later found to be unlawful by the Supreme Court, had reduced the time available for Brexit discussions.
The latest plan comes after the SNP failed to win support for tabling a no-confidence motion with a view to forming an interim government if they succeeded in toppling Johnson.
Under the proposal, the temporary administration would have sought an extension from the EU and then immediately called a general election.
However the other parties were wary, amid concerns Johnson could have tried to hang on in No 10 and delay an election until after October 31 by which time Britain would be out.
There were also sharp differences between the parties over who would lead such a government.
Jeremy Corbyn has said that, as the leader of the biggest opposition party in the Commons, he should become the caretaker PM.
While the SNP broadly supports this, Jo Swinson has been adamant that she would not be prepared to put the Labour leader in No 10.
Her view is shared by many of the rebel Tory MPs who lost the whip after rebelling over Brexit and whose votes would be needed if the plan was to succeed.
Other MPs have proposed grandees such as ex-Tory Ken Clarke or Labour’s Dame Margaret Beckett as caretakers, on the grounds that they would be less divisive.
Other ideas floating around Westminster include tabling another bill to enable the Commons speaker to seek an extension from Brussels if Johnson refuses to do so.
Meanwhile, ministers insist they can still get an agreement with the EU which would allow Britain to leave on October 31 with a deal – despite repeated warnings from Brussels that nothing substantive has been proposed.
Diplomats in the Belgian capital have said they expect the British side finally to table its proposals for resolving the issue of the Northern Ireland backstop once the Tory conference finishes on Wednesday.