U.K. Lawmakers Disrupt Boris Johnson’s Brexit Plan

Opponents of the plan accused Mr. Johnson of negotiating a shoddy deal that would leave a post-Brexit Britain vulnerable to predatory trade deals with other countries, not least the United States.

“This deal would inevitably lead to a Trump trade deal, forcing the U.K. to diverge from the highest standards and expose our families to chlorine-washed chicken and hormone-treated beef,” said the leader of the Labour Party, Jeremy Corbyn, referring to fears of chemically-treated imports from the United States.

For Mr. Johnson, 55, a flamboyant politician and former mayor of London who has been in office since July, it was a crucial moment. He spoke with a tone of gravity and conciliation that contrasted starkly with the inflammatory language he has used during previous parliamentary debates over Brexit.

A victory could fuel Mr. Johnson’s Conservative Party in the general election he is likely to call for the coming weeks. A further delay could paint Mr. Johnson as a leader stymied by Brexit, much like his predecessor, Theresa May, who lost three thumping votes in Parliament on her Brexit agreement.

For Mr. Johnson, the arithmetic was always going to be daunting. To win, he would need to cobble together a complex, contradictory collection of lawmakers, including converts from the opposition Labour Party, hard-line Brexiteers who rejected Mrs. May’s agreement, and Conservative Party exiles whom he purged after they voted for the amendment that defanged his threat to leave the European Union, even without a deal, on Oct. 31.

Mr. Johnson’s deal differs from those of his predecessor, Theresa May, primarily in its treatment of the border issue in Northern Ireland. Needing to avoid physical border checks, Mrs. May opted to keep the entire United Kingdom in the European Union’s customs union, which was unacceptable to hard-line Brexiteers.

Mr. Johnson sought to satisfy them by keeping Northern Ireland subject to the bloc’s rules in a practical sense, but legally outside it with the rest of Britain.