Jeremy Corbyn has said his party will put forward a motion of no confidence "when we can win it" to prevent a no-deal Brexit.
The Labour leader said it would happen at an “appropriate, very early time” but gave no specific detail as to when this might be.
"The Prime Minister seems to be trying to slip no-deal through, slip past Parliament and slip past the British people," he said.
"I'm sorry, it's not on, it's not acceptable. We will do everything we can to block it."
It comes amid warnings that MPs have left it too late to prevent Britain withdrawing from the EU without a deal.
Health secretary Matt Hancock told rebel Tory MPs they could not stop the PM pursuing no deal, saying they had had their chance in the series of Commons votes last month.
His words echoed those of the Prime Minister’s aide Dominic Cummings, who reportedly reminded MPs that, even if the government lost a no confidence vote, Mr Johnson could delay an election until after the Brexit deadline.
However, some senior politicians still maintain that there are options open to Parliament to prevent no deal.
Meanwhile, the Prime Minister Boris Johnson said calling a general election is “the last thing” he wants to do.
"The people of the UK voted in the election 2015, they had a referendum in 2016, and another election in 2017," he said.
"They want us to deliver what they asked for - and that is for us to leave the EU."