DANIEL LEAL-OLIVAS/AFP/Getty

U.K. lawmakers voted Saturday to delay approval of Prime Minister Boris Johnson's Brexit deal, setting up a potentially unprecedented clash between the country's ruling party and the Supreme Court.

The Government was defeated in a crucial Saturday vote, the first weekend sitting in nearly 40 years, by a majority of 16 as lawmakers opted to approve an amendment that effectively forces Johnson to seek an extension to Britain's October 31 EU departure date. Johnson had vowed not to do so, and repeated that assertion again today, setting up a possible legal challenge to his authority in the coming days.

"I will not negotiate a delay with the EU and neither does the law compel me to do so," Johnson told lawmakers following the vote. "I will tell our friends and colleagues in the EU exactly what I have told everyone else in the last 88 days that I have served as Prime Minister -- that further delay would be bad for the country, bad for the European Union and bad for democracy."

Today's vote compels Johnson to send an extension letter request to the EU by 11 pm London time this evening, according to terms of the so-called Benn Act, which was passed by Parliament last month.

There is no guarantee that the EU would accept this request, although experts have said it is highly likely they will in order to give British lawmakers yet more time to agree terms of Britain's EU exit, which was first established by the June 2016 referendum. 

�������� @EU_Commission takes note of the vote in the House of Commons today on the so-called #Letwin Amendment meaning that the #WithdrawalAgreement itself was not put to vote today. It will be for the UK government to inform us about the next steps as soon as possible.

— Mina Andreeva (@Mina_Andreeva) October 19, 2019

The immediate impact, however, is that Britain's EU departure is now once again in question, given that Johnson's deal may not face a full Parliamentary vote until early next week. 

Johnson had planned to put his Brexit deal, reached last week with EU officials in Brussels, to British lawmakers today, with Parliamentary experts suggesting he could just about gather the 320 votes needed for its passage.

However, U.K. lawmaker Oliver Letwin introduced an amendment to the proceedings last week, based on the premise that the deal itself should not be debated in the House until all the relevant legislation attached to it had been passed, lest the deal get approved but its mechanics fail.

Letwin called his amendment "an insurance policy which prevents the UK from crashing out on 31 October by mistake if something goes wrong during the passage of the implementing legislation".

Having lost that vote, Johnson immediately cancelled his plans in order to re-group and study his Parliamentary options amid the legislatures arcane rules. 

House Speaker John Bercow, who decides which debates are allowed to proceed, has said he will give "full consideration" to the government's plan to bring Johnson's deal to the House on Monday, given that lawmakers have voted for a delay until the legislation has been passed.