The Liberal Democrat's new leader has called on Jeremy Corbyn to table a no-confidence motion in the new Prime Minister.
Jo Swinson made the plea in a letter to the opposition leader, who is the only person who can launch such a parliamentary showdown.
She said it is time Mr Corbyn “stops aiding and abetting this Conservative Brexit” and takes action.
“His [Mr Johnson's] 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,” she wrote.
“You must not sit back and allow this government to crash our country out of the EU."
Ms Swinson has tabled a symbolic motion expressing no-confidence in Mr Johnson, but this will not be voted on by MPs.
Earlier this week Mr Corbyn said he plans to table a motion of no-confidence in the Prime Minister “when appropriate to do so”, adding: “It will be an interesting surprise”.
However Labour had previously called a no-confidence vote a “nuclear option” that would only strengthen Mr Johnson’s position.
A Labour spokesman said: “This is childish and irresponsible game-playing by the Lib Dems who are more interested in attacking Labour than stopping no deal.”
Today is the final day in which Mr Corbyn could launch a no-confidence vote before the House of Commons breaks for summer recess.
If he were to table a motion, it would be debated and voted upon on September 3.
A successful no-confidence vote would lead to either a new administration or an election.