London: British Prime Minister Boris Johnson on Tuesday lost his parliamentary majority, ahead of a crucial vote sought by a group of MPs opposing his "no-deal" Brexit, reports said.

Johnson, who succeeded Theresa May as Prime Minister in July this year, had a working majority of just one in the 650-member House of Commons, but even lost that as party MP Philip Lee defected to the Liberal Democrats on Tuesday.

Lee went to sit on the opposition benches amid their cheers even as Johnson was addressing the House, the BBC reported. A woman Conservative MP had also shifted sides last month.

Explaining his decision, Lee, the member from Bracknell and who had voted to remain in the EU in the 206 referendum, charged the government with "pursuing a damaging Brexit in unprincipled ways", putting lives and livelihoods at risk.

In a letter to Johnson, Lee said that the rift over Brexit had "sadly transformed this once great party into something more akin to a narrow faction in which one's Conservatism is measured by how recklessly one wants to leave the European Union" and that it had "become infected by the twin diseases of English nationalism and populism".

He said the Liberal Democrats - who back another referendum on Brexit and want the UK to remain in the EU - were best placed to "heal the divisions".

The government has now 319 members - the Conservatives with 309, and 10 of its Northern Ireland ally, the Democratic Unionist Party.

The opposition benches comprise 320, of which the principal opposition, the Labour Party, has 245, followed by the Scottish National Party with 35 and the Liberal-Democrats, which now number 15. Smaller parties, such as The Independent Group for Change and Plaid Cymru, account for the other 25 seats.

The Speaker and his three deputies do not vote, while the Sinn Fein's 7 members abstain from proceedings.

The recently-installed Johnson government's loss of majority comes as a group of MPs, cutting across party lines and opposing his move to leave the EU on October 31 even if there is no deal, submitted a motion for an emergency debate to Speaker John Bercow, who allowed it.

That debate could last up to three hours, followed by a vote.

If the government is defeated in the vote, the MPs will be able to control the house's business and bring in a bill that would force the Prime Minister to ask for Brexit to be delayed until January 31, unless the house approves a new deal, or vote in favour of a no-deal exit, by October 19, the BBC reported.

Even as Lee was shifting sides, the Prime Minister told the house that he wanted a negotiated exit from the EU but the moves by the MPs to block a no-deal departure would "destroy any chance of negotiating a new deal".

Johnson has hinted at a snap election if he loses the vote.