British Prime Minister Boris Johnson braced for another showdown in parliament on Wednesday after a humiliating defeat over his Brexit strategy, with MPs set to vote on a law aimed at blocking a no-deal departure.

Johnson has said he will seek an early general election if MPs vote against him again, intensifying a dramatic political crisis ahead of his October 31 Brexit deadline.

Johnson has vowed to take Britain out of the EU on October 31 whether or not a divorce deal with Brussels is in place, more than three years after the referendum vote to leave the European Union.

But his opponents warn that Brexit without a deal will have disastrous economic consequences and must be avoided.

In a sign of the government's determination, finance minister Sajid Javid will also on Wednesday unveil another £2 billion (2.2 billion euros, $2.4 billion) of funding to deal with Brexit, including for new port infrastructure.

But Johnson's Conservative government is in disarray.

It lost its working majority in parliament on Tuesday after one of its MPs switched to the anti-Brexit Liberal Democrats and, a few hours later, it expelled 21 MPs from the party for voting against the government.

"Humiliation for Johnson as Tory rebels turn against him," read the front-page of the left-wing Guardian newspaper, while The Independent wrote: "Johnson loses control".

But the strongly eurosceptic Daily Express said rebel MPs had voted "to betray Brexit" and called Tuesday's vote "another shameful day in our so-called democracy".

The rebels included Conservative Party grandees such as Ken Clarke, the longest-serving member of parliament, and Nicholas Soames, Winston Churchill's grandson.

They joined with opposition lawmakers to inflict a first blow against Johnson's hardline Brexit strategy.

That opened the way for a vote on Wednesday on a law which would force Johnson to delay Brexit by three months if he does not strike a deal with the EU by October 19.

MPs are due to finish voting on the law by around 1800 GMT.

After a lengthy and heated session of parliament, Johnson said that the proposed legislation would undermine his discussions with Brussels on a new divorce deal and would cause "more dither, more delay and more confusion".

But Johnson's critics say his claim that he wants a new deal with Brussels is a sham and the EU has said Britain is yet to come forward with "any concrete proposals".

"I don't want an election but if MPs vote tomorrow to stop the negotiations and to compel another pointless delay of Brexit, potentially for years, then that will be the only way to resolve this," Johnson said on Tuesday.