LONDON—Boris Johnson was left raging on Saturday as lawmakers forced the prime minister to seek yet another Brexit delay from the European Union.

The extremely rare parliamentary vote taken on a Saturday did not reject Johnson’s compromise deal with the EU outright, it merely demanded more time for the deal to be examined and inserted an additional failsafe to stop Britain from slipping out of the EU without an agreed deal on Halloween.

No. 10 was furious because Johnson has repeatedly promised to leave the EU by October 31, and that will now become more difficult. Brexit campaign insiders lamented the destruction of Johnson’s “head of steam,” and an end to the momentum created by his unlikely success in securing a deal from Europe.

After another vote that went against Johnson last month, the prime minister is now legally mandated to write to the EU asking for an extension to January 31. It is unclear how Brussels will respond to that request.

Johnson is expected to bring the withdrawal legislation to the floor of the House of Commons early next week, so he may only have to wait a few days to secure victory but Labour opponents—and nervous No. 10 insiders—believe that potential support for the deal may ebb away once lawmakers get the chance to fully examine the fineprint.

Just two days after Johnson was back-slapping European counterparts and clasping hands with fellow leaders, his precarious grip on power was underlined once again in a vote that went against him by 322 to 306.

In response, Johnson stood up and said he would refuse to “negotiate” a further extension with the EU. He stopped short of saying he would refuse to comply with the law and send the extension letter, although he reiterated his hopes that the EU would not immediately grant an extension. “I don't think they'll be attracted by delay,” he said.

As lawmakers continued to debate the result, Johnson sat slumped on the frontbench shaking his head.

It was a sharp contrast to his mood two days earlier. Tickled pink with the deal he had unexpectedly secured from the EU, Johnson had sought to rush back to Westminster and bounce parliament into agreeing.

One of his own long-term colleagues, Sir Oliver Letwin, had other ideas.

Letwin is a veteran Conservative right-winger who has been in the heart of Conservative thinking for decades. He was a member of Margaret Thatcher’s Downing Street policy unit in the 1980s and entrusted by David Cameron to write the Tory manifesto in 2010.

He was kicked out of the party last month by Johnson after voting to ensure there wouldn’t be a No Deal Brexit.

He exacted his revenge on Saturday by wrecking Johnson’s chance for a victorious homecoming.