Prime Minister Boris Johnson on Tuesday lost his working majority in parliament with the dramatic defection of a party member ahead of a showdown with lawmakers over Brexit that could lead to an early general election.

In a heated parliamentary session, Johnson condemned a plan by lawmakers to block his Brexit strategy as "surrender" and said it would undermine his intention to negotiate a new divorce deal with the EU.

Opposition lawmakers and rebel members of his Conservative party are planning to vote for delaying beyond October 31 if he cannot agree exit terms with Brussels.

While Johnson was making his statement, Conservative lawmaker Phillip Lee was seen crossing the floor of the Commons to sit with the pro-European Liberal Democrats.

Lee said in his resignation letter that the Conservative Party "has become infected with the twin diseases of populism and English nationalism" as a result of Brexit.

Lawmakers will first try to make room in the parliamentary agenda for a debate of the bill by putting forward a motion set to be voted on by lawmakers on Tuesday evening.

If they succeed, they will introduce their bill on Wednesday and seek to get it through before parliament is suspended next week.

Johnson's aides have warned that defeat in a first vote in the House of Commons expected at 2000 GMT on Tuesday would force him to call a snap election on October 14.

Johnson would need the support of the main opposition Labour party to call a snap poll, as the law requires the backing of two-thirds of lawmakers.

Labour leader Jeremy Corbyn said Johnson was not really intending to do a deal with Brussels and instead planned to crash Britain out of the EU.

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

Pound tumbles

The rebels believe they have the numbers to force through the plan, which is backed by the main opposition Labour party and could delay Brexit to January 31.

Lee's defection means the prime minister no longer has a majority in the 650-seat chamber.

The government's numbers could shrink even further if it dismisses lawmakers that vote against it later on Tuesday.

But losing the majority does not automatically bring down the government as this can only happen if the government loses a formal confidence vote.

On a day of high drama, an Edinburgh court also heard a legal challenge against Johnson's decision to suspend parliament next week for more than a month, which critics said was a bid to silence lawmakers.

The judge is expected to announce his ruling on Wednesday.

The heightened political tension sent the British pound tumbling on Tuesday to its lowest level against the dollar in almost three years.

$16 billion lost

Johnson took office less than six weeks ago, after his predecessor Theresa May was forced out over her failure to get her Brexit divorce deal through parliament.

From the start, he faced opposition from his own lawmakers who fear his threat of leaving the EU without an agreement with Brussels risks severe economic disruption.

Leaked government assessments have warned that no-deal could lead to food, fuel and medicine shortages.

U.N. economists also on Tuesday warned that Britain could lose at least $16 billion (14.6 billion euros) a year on exports to the European Union if it left without a deal.

'No concrete proposals'

Johnson has rejected the divorce deal on the table but insists he wants to reach an agreement with Brussels to ease the end of Britain's 46-year-old EU membership.

EU leaders have refused to reopen the current Brexit text but Johnson insists progress is being made, saying that only with a credible threat to walk away will he secure a new deal.

But critics note that there are no formal negotiations with Brussels, and both sides have stepped up preparations for a disorderly divorce next month.

A spokeswoman for the European Commission said Tuesday that it had yet to see any "concrete proposals" from London on how it wants to change the existing deal.

Corbyn has repeatedly called for an election and warned that if the legislative route fails, he may try to force one by calling a confidence vote in the government.

But many Labour lawmakers also fear a trap.

Johnson's aides insist any election would be held before Brexit, but some of his opponents fear he could change the date at the last minute to after October 31.

This would leave parliament powerless to stop a "no deal" Brexit.