Prime Minister Boris Johnson on Tuesday dramatically lost his majority in the House of Commons when the Conservative MP Phillip Lee crossed the floor of the chamber to join the Liberal Democrats.

The prime minister appeared flustered as he watched Lee take a seat on the opposition benches.

The defection means that Johnson's government has lost its working majority in the Commons.

The prime minister has threatened to expel dozens of Conservative MPs if they join a rebellion against the government on Brexit on Tuesday evening.

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The Conservative MP Phillip Lee on Tuesday left his party and joined the Liberal Democrats in a dramatic move that stripped Prime Minister Boris Johnson of his majority in Parliament.

Lee deserted the Conservatives in the most public way possible, crossing the floor of the House of Commons while the prime minister was speaking after Parliament returned from the summer recess.

Opposition MPs were quiet for a moment as Lee took his seat next to Jo Swinson, the Liberal Democrat leader, before cheers broke out.

Watch Lee cross the floor to join the Liberal Democrats:

Lee had for months been widely touted as a defector, and he had acknowledged that he was considering joining the Liberal Democrats after calling himself "politically homeless" in the Conservative Party because of his opposition to Brexit and his support for a second EU referendum.

Read more: Boris Johnson will seek a general election if Parliament votes to block a no-deal Brexit

The move means Johnson is now effectively unable to pass any legislation in the Commons without the support of opposition MPs — which is rarely forthcoming — and further increases the prospect of a general election, which could be triggered as early as this week.

In a statement, Lee accused Johnson's government of "using political manipulation, bullying and lies."

He added: "This Conservative Government is aggressively pursuing a damaging Brexit in unprincipled ways. It is putting lives and livelihoods at risk unnecessarily and it is wantonly endangering the integrity of the United Kingdom."

Several other Conservative MPs announced on Tuesday that they would resign at the next election in protest of Johnson's Brexit strategy.

The prime minister has threatened to expel any Conservative member of Parliament from the party if they join with the opposition on Tuesday evening to vote for a Brexit delay to prevent a no-deal exit in October.