U.K. lawmakers voted on Saturday to undermine a deal reached between Boris Johnson and the European Union.
Interested in Brexit?Add Brexit as an interest to stay up to date on the latest Brexit news, video, and analysis from ABC News.
Hours later, the British government formally asked the European Union to extend its Brexit deadline past Oct. 31, though the communication also included a letter from Johnson making clear that he disagrees with the request.
Johnson announced Thursday that he had reached a deal with European leaders for the U.K. to leave the EU, but the agreement still needed to be approved by his country's Parliament.
But in the latest stunning rebuke, lawmakers on Saturday approved an amendment, by a vote of 322 to 306, that said Parliament must pass all of its Brexit-related bills before a deal can be reached with the EU.
Meanwhile, the U.K. Parliament had gathered to vote on Johnson's deal -- the first time the body gathered on a Saturday in nearly four decades. The vote has now been rescheduled for next week.
Johnson has repeatedly ruled out asking EU leaders for an extension to the Brexit deadline, and vowed to continue with his Brexit plans "unchanged" despite the setback.
"I will not negotiate a delay with the EU, and neither does the law compel me to do so," Johnson said after the vote. "I will tell our friends and colleagues in the EU exactly what I’ve told everyone in the last 88 days that I’ve served as prime minister: that further delay would be bad for this country, bad for the European Union and bad for democracy."
Johnson said his government will introduce legislation next week "for us to leave the EU with our new deal on Oct. 31."
The leader of the opposition Labour Party, Jeremy Corbyn, praised Parliament's vote as a mechanism to “stop a no-deal crash out from the European Union” and said the prime minister must comply with the law.
As the vote took place hundreds of thousands of protesters marched through London to call for a second Brexit referendum.
In early September lawmakers voted 327 to 299 in favor of a law that will force the prime minister to ask for an extension to the Brexit deadline if a deal has not been passed. The amendment effectively builds into the law another insurance policy to avoid a "no-deal" Brexit.
That law means Johnson is now required to write a letter requesting an extension to the Oct. 31 Brexit deadline by midnight on Saturday. The EU is expected to reluctantly agree to an extension, despite Johnson saying after today's defeat he "would not negotiate" a deal with EU leaders.
If no extension is agreed with EU leaders, the default procedure means the U.K. is still scheduled to leave the EU without a deal after the Oct. 31 deadline. Critics say such a development could be devastating hugely to the UK economy.