Parliamentary leaders of four opposition parties have written to Jeremy Corbyn calling on the Labour leader to join them in forcing an immediate vote of no confidence in Theresa May as Prime Minister.
The letter, signed by Liberal Democrat leader Sir Vince Cable, Green MP Caroline Lucas, SNP leader in Westminster Ian Blackford and Plaid Cymru Westminster Group leader Liz Saville Roberts, says there is an "overwhelming" case for a confidence motion following Mrs May's decision to defer the crunch Commons vote on her Brexit deal.
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They said it was for Labour, as the main party of opposition, to take the initiative in bringing forward a vote which could topple Mrs May and pave the way for a second EU referendum.
Mr Corbyn has repeatedly made clear that he wants an early general election to allow Labour a chance to take over the Brexit negotiation process.
But he has so far held back from tabling a no confidence motion under the 2011 Fixed-Term Parliaments Act, which requires a majority of MPs to pass.
Under the terms of the Act, a general election is triggered 14 days after a successful no confidence motion, unless MPs express their confidence in the Prime Minister or a successor in a subsequent vote within that time.
A Labour spokesman said on Monday that the party would table a motion "when we judge it most likely to be successful", indicating that this would probably be when Mrs May returns from Brussels with whatever reassurances she has been able to obtain from the EU.
Mr Corbyn has called a debate in the Commons on Tuesday on the PM's "unprecedented" decision to call off the meaningful vote without the agreement of the House.
The letter from opposition parties said that Labour's shadow cabinet was meeting on Tuesday "at a time of constitutional and national crisis".
"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 call for a motion of no confidence to be put on the floor of the House of Commons," it said.
"Labour's annual conference in September voted for a policy that commits your party to oppose a bad Brexit deal, seek a general election and - if that does not succeed - campaign for a People's Vote.
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"Today is your opportunity to begin fulfilling that policy by joining the Westminster Parliamentary leaders of the Scottish National Party, the Liberal Democrats, Plaid Cymru and the Green Party in supporting a motion of no confidence in the Government.
"We believe a motion of no confidence must be brought forward at the earliest possible opportunity, to ensure there is enough time to pursue another course. Therefore we want to emphasise again that you will have our full support if you put down a motion of no confidence."
The four were due to appear before the cameras at a press conference hosted by the People's Vote campaign later in the day to spell out their case for a motion "to enable confidence in this Government to be tested before it is too late and to put Parliament back in charge of this process".
A Labour spokesman said on Monday: "We will put down a motion of no confidence when we judge it most likely to be successful.
"It is clear to us that Theresa May will not renegotiate the deal when she goes to Brussels, and will only be asking for reassurances from EU leaders.
"When she brings the same deal back to the House of Commons without significant changes, others across the House will be faced with that reality.
"At that point, she will have decisively and unquestionably lost the confidence of Parliament on the most important issue facing the country, and Parliament will be more likely to bring about the general election our country needs to end this damaging deadlock."
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