Around 160 MPs have signed a declaration pledging to stop a no-deal Brexit "using whatever mechanism possible".
Opposition leaders signed the declaration at Church House in Westminster - the meeting place of the Houses of Parliament during the Second World War and the proposed site of a ‘rebel’ parliament in the event that Boris Johnson suspends the Commons to force through a no-deal.
Labour was represented by shadow Chancellor John McDonnell, who said the Prime Minister was a "threat to democracy" for not ruling out proroguing Parliament.
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Mr McDonnell said the leader of the opposition Jeremy Corbyn was “trapped in meetings”.
The declaration was also signed by SNP Westminster leader Ian Blackford, Liberal Democrat leader Jo Swinson, Plaid Cymru Commons leader Liz Saville Roberts, Green Party MP Caroline Lucas and Change UK leader Anna Soubry.
Independent MP Luciana Berger and Labour MP Stephen Doughty, who organised the declaration, said it already has 160 signatures from MPs of all parties except the DUP.
Ms Lucas said that any attempt to suspend parliament over Brexit would be “nothing less than a coup”.
And Mr McDonnell claimed: “Prime Ministers come and Prime Ministers go, but I don’t think we have seen a Prime Minister like this who has had the potential to threaten the very nature of our democracy.”
Ms Soubry, a former Conservative MP, hit out at parliamentary colleagues who have backed the government despite opposing Brexit.
“You all know who the people of courage are and those who have failed to exercise it,” she said.
“They will stop you in the corridors and say: ‘of course this is absolute madness’ but they will not go and do the right thing, which is to be true to their principles.”
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The text of the "Church House Declaration" reads: "Shutting down Parliament would be an undemocratic outrage at such a crucial moment for our country, and a historic constitutional crisis.
"Any attempt to prevent Parliament sitting, to force through a no-deal Brexit, will be met by strong and widespread democratic resistance.
"We pledge to work together across parties and across our nations to do whatever is necessary to ensure that the people's voice is able to be heard."