After British MPs voted on Saturday to delay a decision on whether to approve Boris Johnson's Brexit deal, the prime minister is now bound by the so-called "Benn Act" to ask the EU for an extension to the divorce deadline.
The law, named after opposition Labour MP Hilary Benn, compels Johnson to deliver a letter to EU Council president Donald Tusk to formally request a delay, which he has vowed not to do.
Here are the main points in the legislation which Johnson calls a "surrender" document.
The bill states that if parliament does not approve a separation deal with the EU by Saturday, the prime minister must ask to delay Brexit until January 31, 2020.
"The purpose of the bill is to ensure that the UK does not leave the European Union on the 31st of October without an agreement, unless parliament consents," Benn said at the time.
Johnson had hoped to get his deal approved during Saturday's special sitting of parliament, thereby nullifying the legislation.
But MPs voted on an amendment that delayed the official vote, activating the Benn bill.
The prime minister had previously said he would rather be "dead in a ditch" than ask for another delay, and immediately after Saturday's vote said he would not negotiate an extension, leaving many scratching their heads about what he planned to do.
The legislation includes a template of the letter Johnson should write to Tusk by midnight Saturday (2300 GMT) to formally request a delay to Brexit.
"The United Kingdom is seeking a further extension," the letter states, adding that the Brexit date could be brought forward if a deal is ratified before January 31.
Under the law, the prime minister would have to immediately accept an offer from EU leaders to delay Brexit until January 31.
If the extension offered by the EU is to a different date, the prime minister would have two days in which to accept.
The only way of not accepting a delay in that case would be if parliament votes against it within those two days.
Proponents of Saturday's amendment argued that although Johnson brought back a deal that stood a good chance of being approved, MPs may need beyond October 31 in order to pass all the legislation required to formalise Brexit.
They fear that if they don't have enough time to scrutinise the formal legislation before the end of the month, Britain will leave the EU without a deal by accident.
The amendment was described as an "insurance policy" against that happening.
If there is still no agreement by January 31, the Benn Act implies that Brexit would have to be delayed again.
Johnson has warned that the legislation could have the effect of delaying Brexit "potentially for years".