Boris Johnson, the British prime minister, lost his working majority in Parliament Tuesday after a member of his Conservative party defected to the rival Liberal Democrats as he tries to force through a no-deal Brexit.

Phillip Lee crossed the floor of the House of Commons just as Johnson was giving a statement on last month's G7 summit.

He will sit with the Lib Dems, a party that opposes Brexit and wants to remain in the European Union.

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Boris Johnson lost his working majority in Parliament Tuesday after a member of his Conservative party defected to the rival Liberal Democrats as he tries to force through a no-deal Brexit (AP)

“I have reached the conclusion that it is not possible to serve my constituents’ and country’s best interests as a Conservative Member of Parliament,” Lee said in a statement.

“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.”

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His decision will weaken Johnson as he tries to prevent Parliament from blocking his Brexit plans. Even with the support of Northern Ireland's 10 Democratic Unionist Party MPs, Johnson can't rely on passing laws or votes in the House of Commons. The Conservatives and the DUP have a combined 319 MPs, while the Opposition, made up Labour, the Scottish National Party, Liberal Democrats and others number 320. There are 11 MPs who don't traditionally vote, including the Speaker of the House.

Moments later, Jeremy Corbyn, leader of the opposition Labour Party, attack Johnson for trying to run a "cabal".

"This is a government with no mandate, no morals, and as of today, no majority," he said.

Pro-EU supporters protest outside the Houses of Parliament in London, Tuesday, Sept. 3, 2019. (AP)

Corbyn is playing a central role in a parliamentary bid to seize control of the agenda and pass legislation that would force Johnson to seek a delay to the proposed Oct. 31 Brexit deadline.

Rebels within Johnson's party are also threatening to force the government to seek a delay. Earlier Tuesday, Johnson held talks with the rebels, telling them, "You are handing power over to a junta that includes Jeremy Corbyn," according to The Sun.

But Johnson is expected to call an early general election, two weeks before the Halloween deadline, if he is ordered by Parliament to delay it again.

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All the uncertainty and turmoil meant that the pound on Tuesday touched its lowest level against the U.S. dollar since 1985, excluding a brief "flash crash" in 2016 that might have been the result of technical glitches.

The Associated Press contributed to this report.