Around 160 UK Members of Parliament (MPs) from the opposition parties signed a declaration of unity to prevent a no-deal Brexit at London's Church House on Tuesday.
Shadow chancellor John McDonnell, Liberal Democrat leader Jo Swinson, SNP Westminster leader Ian Blackford, Green Party leader Caroline Lucas and Change UK leader Anna Soubry were among those present at the ceremony.
""I just give this warning to Boris Johnson: Prime Ministers come and Prime Ministers go but actually I don't think we've ever seen a Prime Minister like this who has the potential to threaten the very nature of our democracy. And I just warn him that we will not stand aside and let that happen," McDonnell said.
"We will use whatever mechanism necessary to prevent that happening to secure the democratic rights of our constituents to be heard through their MPs but also to secure the future of our country against such a reckless act," the Shadow Chancellor added.
Tuesday's declaration comes as Labour Party leader Jeremy Corbyn said he had contacted 116 Conservative and independent MPs, including former Tory leader Theresa May and Philip Hammond, asking them to join efforts to prevent a no-deal Brexit.