Jo Swinson, the Liberal Democrat leader, put forward a no-confidence motion in Mr Johnson only hours after he entered No 10, expressing alarm at the prime minister’s enthusiasm for a no-deal Brexit.
The Lib Dem motion reads: “That this house has no confidence in the prime minister; rejects the option of the UK crashing out of the EU; and rejects the option of parliament being prorogued before Friday 8 November 2019.”
While the motion is non-binding, it ramps up pressure on Labour to bring an official challenge against the prime minister – as only the leader of the opposition is allowed to do so.
In a letter to Mr Corbyn, Ms Swinson said: “Boris Johnson does not hold a mandate from parliament or the general public to be prime minister.
“His reckless refusal to rule out proroguing parliament in order to crash the UK out of the EU without a deal demonstrates that he is not fit to lead this country.
“As the leader of the official opposition you are the only person in the position to call a vote of no confidence in the new prime minister.
“We therefore urge you to do so immediately. You must not sit back and allow this government to crash our country out of the EU.
“We implore you to take action and call for a vote of no confidence. It is vital that we work together to secure a people’s vote with the option to stay in the EU.”
But Labour refused to act on her challenge, accusing the Liberal Democrats of “childish and irresponsible game playing”.
A spokesperson said: “As Jo Swinson well knows, a no-confidence vote now will only strengthen Boris Johnson’s hand and further his march towards no deal.
“This is childish and irresponsible game playing by the Lib Dems, who are more interested in attacking Labour than stopping no deal.”
Senior Labour figures have refused to set a date for a new confidence vote, as the party wants to save its firepower for the autumn when it has a genuine chance of success.
John McDonnell, the shadow chancellor, predicted that the best moment would be when “Boris comes back with something that’s undeliverable or unacceptable around Brexit”.
He told ITV’s Peston: “And then you judge it in the conversations you’ll have across the house about what’s feasible and what’s not.
“You’ve got large numbers of [Tory] people who are going to be on the back benches now, not particularly happy, and also I think quite shocked like the rest of us about how far Boris has gone.”
The row comes as Mr Johnson began his premiership with a bloody cabinet reshuffle, clearing out Theresa May’s allies and appointing a string of staunch Brexiteers and Vote Leave veterans to key posts.
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