LONDON—Boris Johnson’s “do or die” pledge to take Britain out of the European Union by Oct. 31 was quashed by Parliament on Tuesday night, handing the initiative to the EU to effectively trigger a British election.

Johnson threatened to call for a general election if British lawmakers refused to allow him to rush through his deal and the EU proposed a new extension of three months or more. Under a law passed in Westminster last month, Johnson is not allowed to negotiate to shorten whatever extension the EU chooses.

Rather than seek to compromise with opponents who want proper time to scrutinize the Brexit deal, Johnson responded to the 322 to 308 vote defeat on fast-tracking it by halting the passage of his deal altogether while Britain waits to see what extension the EU will grant.

“We will pause this legislation,” he said, a phrase that sounded innocuous but could well kick-start an epic new election showdown between the forces of Remain and Leave.

A snap vote could take place before Christmas.

It remains to be seen if Johnson is as good as his word—and there have been plenty of reasons to cast doubt on it in the past—as there was no specific mention of the election he had threatened earlier in the day in the aftermath of his defeat. Under Britain’s fixed-term parliament act, a two-thirds majority is usually required to call an election so both the government and the opposition would have to agree.

Donald Tusk, president of the European Council, did not need a second invitation, however. Soon after Johnson said he was pausing the agreement, Tusk tweeted that he was talking to leaders in Brussels about issuing a written extension, which is likely to delay the Brexit deadline until January 31.

If that were confirmed, British politicians would be under heavy pressure to agree to hold an election and seek a fresh mandate from the voters before proceeding with any Brexit deal.

Johnson had earlier won a vote on his deal—the first time his government has won a single significant vote in the Commons. That was a major step towards securing Brexit, as Parliament has always refused to back any formal arrangement that would result in leaving the EU.

The next phase of the legislation’s progress is where things become more difficult, however, as lawmakers are able to amend the bill in order to clarify sections or—as No. 10 fears—introduce so-called wrecking amendments that would collapse the bill entirely.

Just last week, Johnson had secured a compromise deal that many thought was impossible in Brussels, but that came at a serious cost. The EU had sworn they would not re-open the Withdrawal Agreement that had been negotiated with Theresa May, but then Johnson did what he said he would never do and he caved on one of his key red lines.

He signed up to a version of the deal that May had rejected, which would effectively create a customs border in the Irish Sea between Northern Ireland and the mainland.

That concession led to a breakthrough in Europe but it meant the Democratic Unionist Party, which had been propping up the Conservative government, fled from the deal.

It was lawmakers who made the most aggressive speeches attacking the prime minister during a contentious debate in the House of Commons. Sammy Wilson of the DUP said he felt they had been betrayed by the Conservatives. “I nearly choked when the prime minister said it,” he said on Tuesday.

Wilson and his nine DUP colleagues voted against Johnson’s expedited deal.

Wilson was particularly aggravated that Johnson had been unfamiliar with the precise details of the deal he had agreed that would govern Northern Ireland’s relationship with the rest of Britain.

There were doubts about exactly how familiar Johnson was with the customs rules that he was attempting to rush through Parliament.

Jill Rutter, an independent former civil servant who worked at the Treasury and No. 10, said: “I don’t think Johnson understands what he has agreed for Northern Ireland…”

With the Europeans jumping on his “pause” to bind Britain into another extension, Johnson may have also misunderstood that he was putting his job on the line.