U.K. Lawmakers Approve Brexit Trade Deal, Cementing EU Exit

The tight timetable reflects the need to ratify the agreement before the end of the year, or risk economic disruption and legal uncertainty as a yearlong transition period to smooth the U.K.’s EU exit comes to an end. Many lawmakers opposed to Brexit said they would vote reluctantly in favor of the U.K.-EU treaty, as the only alternative is a messy close to the saga.

Speaking in Parliament at the opening of Wednesday’s debate, Prime Minister Boris Johnson said the treaty represented a new beginning for the U.K. and the EU.

“What we wanted was not a rupture but a resolution—a resolution of the old, tired, vexed question of Britain’s political relations with Europe, which has bedeviled our postwar history,” he said.

Keir Starmer, leader of the main opposition Labour Party, said he would ask his lawmakers to back the agreement to avoid disruption on Jan. 1. He criticized aspects of the accord, including its provisions on services trade and law enforcement, but said it was the only deal available to prevent a damaging no-deal outcome.

“It’s imperfect, it’s thin…But we have only one day before the end of the transition period and it is the only deal that we have,” Mr. Starmer said.

The House of Commons voted 521 to 73 in favor of the legislation to implement the accord.

Some constitutional experts said the short time to scrutinize such a far-reaching treaty was unsatisfactory. Parliament had six weeks to pore over the withdrawal treaty that provided the basis for Brexit in 2019, for instance, and spent months debating major pieces of legislation related to European integration when Britain was a member state.

“This process represents an abdication of Parliament’s constitutional responsibilities to deliver proper scrutiny of the executive and of the law,” said Brigid Fowler, senior researcher at the Hansard Society, a nonpartisan London-based research institute focused on parliamentary democracy.

The accord setting out the U.K.’s future relationship with the EU is in fact three separate agreements. As well as a free-trade accord, the pact includes cooperation agreements on security and nuclear power.

Each agreement was signed Wednesday first in Brussels by Ursula von der Leyen, president of the European Commission—the EU’s executive branch—and later in London by Mr. Johnson. A Royal Air Force aircraft ferried two copies of each agreement between the two capitals. One set of copies will remain in each capital. Charles Michel, president of the European Council, which brings together the leaders of individual member states, also signed the trade and security accords on behalf of EU capitals.

The pact goes into effect provisionally on Jan. 1 because the European Parliament hasn’t had time to scrutinize it. The debate and vote on the agreement hasn’t been scheduled yet.

The treaty was also backed by prominent Conservative “euroskeptics” whose decadeslong crusade against EU membership was central to bringing about Brexit. Iain Duncan Smith, a former Conservative Party leader, said the treaty brings power back to Britain’s elected representatives in London from officials in Brussels.

Of Britain’s European allies, he said, “For too long, we moved into the same house with them and we didn’t get on. We’re now just going to move next door and be good neighbors, good friends and good allies.”

Former Prime Minister Theresa May voted for the deal but said she was disappointed it didn’t include services, a hugely important sector for the British economy.

The treaty was opposed by the Scottish National Party and the Liberal Democrats, two smaller opposition parties that campaigned in favor of remaining in the EU in 2016. It was also opposed by Northern Ireland’s Democratic Unionist Party, onetime allies of Mr. Johnson opposed to new provisions relating to Northern Ireland, which is part of the U.K.

Britain and the EU agreed to a complex regulatory framework for Northern Ireland to reduce the need for border controls between the region and the Republic of Ireland, an EU member state. Fears that Brexit could lead to a physical border between the two parts of the island after a history of political unrest there shaped negotiations from the outset.

—Laurence Norman in Brussels contributed to this article.

Write to Jason Douglas at jason.douglas@wsj.com