Jeremy Corbyn has urged the leaders of the other opposition parties and Tory rebels to install him as caretaker PM in order to stop a no-deal Brexit.
If he wins a no-confidence vote in the government, the Labour leader plans to delay Brexit, call a snap election and campaign for another referendum.
Mr Corbyn outlined his plan in a letter, saying the caretaker government would be “strictly time-limited”.
Liberal Democrat leader Jo Swinson dismissed the plans as “nonsense”.
Downing Street said Mr Corbyn would “overrule the referendum and wreck the economy” if he became prime minister.
Prime Minister Boris Johnson said he wants a deal with the European Union, but insists the UK must leave the bloc by 31 October “do or die”.
Mr Corbyn said he would seek to call a no-confidence vote at the “earliest opportunity when we can be confident of success”.
To defeat the government he would have to secure the backing of a majority of MPs.
In his letter, Mr Corbyn wrote: “This government has no mandate for no-deal, and the 2016 EU referendum provided no mandate for no-deal.
“Following a successful vote of no confidence in the government, I would then, as Leader of the Opposition, seek the confidence of the House for a strictly time-limited temporary government with the aim of calling a general election, and securing the necessary extension of Article 50 to do so.”
If he were to succeed in calling a general election – which would require the support of two-thirds of MPs – Labour would campaign for a second referendum with the option to remain in the EU, he said.