Boris Johnson has been described as a threat to the “very nature” of British democracy at a cross-party meeting where MPs agreed to form an alternative parliament in the event of the prime minister shutting down the existing one to make a no-deal Brexit happen.

In a symbolic gathering at Church House in Westminster, where MPs met during the second world war, Labour’s John McDonnell took to the stage alongside the former Conservative MP Anna Soubry as well as the Liberal Democrat leader, Jo Swinson, and Caroline Lucas of the Green party.

They signed the Church House declaration, which said shutting down parliament would be “an undemocratic outrage at such a crucial moment for our country, and a historic constitutional crisis”.

The pronouncement added: “Any attempt to prevent parliament [from] sitting, to force through a no-deal Brexit, will be met by strong and widespread democratic resistance.”

McDonnell, the shadow chancellor, who said the Labour leader, Jeremy Corbyn, had been stuck in meetings, told those assembled: “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.”

McDonnell said he wanted to warn Johnson that MPs will use “whatever mechanism necessary” to prevent the prime minister from proroguing parliament. Organisers said more than 160 MPs from a range of parties signed the declaration.

Play Video 1:19 Boris Johnson a threat to UK's democracy, says John McDonnell – video

Ian Blackford, the leader of the Scottish National party in the House of Commons, said the government posed a threat to the security of citizens, while Lucas said: “We will block what is nothing less than a coup.”

Swinson said MPs had come together to make the point that they would not stand by while the government pursued a “catastrophic no-deal Brexit”.

Although Conservative MPs were conspicuous by their absence from the event, Soubry, who left the Tories to join the Independent Group for Change, spoke on stage with McDonnell sitting beside her.

She said she hoped the history books would record their determination and courage and show there were those who acted while others stood by and did nothing.

“You all know who the people of courage are and those who have failed to exercise it,” Soubry 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.”