MP Phillip Lee defected from the Conservative Party to join the opposition Liberal Democrats, leaving Johnson’s government without a working majority.
“This Conservative Government is aggressively pursuing a damaging Brexit in unprincipled ways,” Lee said in a statement.
“It is putting lives and livelihoods at risk unnecessarily and it is wantonly endangering the integrity of the United Kingdom. More widely, it is undermining our country’s economy, democracy and role in the world,” he added, before accusing Johnson’s administration of “using manipulation, bullying and lies in a deliberate and considered way.”
Johnson said he would meet Irish Prime Minister Leo Varadkar in Dublin on Monday as part of efforts to reach a deal to leave the European Union (EU).
Addressing parliament after it returned from a summer recess, Johnson said he was making progress in talks with the EU to change an agreement negotiated by his predecessor Theresa May.
He argued there were other ways to avoid any return to a hard border between Northern Ireland, a constituent part of the UK, and EU member Ireland than the existing withdrawal agreement’s controversial so-called “backstop” provision.
European leaders have consistently rejected calls for the safety net to be axed, however, with the bloc’s lead Brexit negotiator Michel Barnier saying over the weekend that the clause was the “maximum flexibility” the EU could offer.