The decision, agreed on by ambassadors from the 27 other EU countries on Monday, grants a three-month extension to Britain’s EU membership. It allows the U.K. to leave earlier if it and the EU both ratify the withdrawal deal that Mr. Johnson agreed with the leaders of the bloc’s member states earlier this month.
The move was announced by European Council President Donald Tusk on Monday and means Brexit has now been extended three times this year. The U.K., which voted in June 2016 to leave the EU, was originally supposed to leave on March 29 but has been unable to ratify a Brexit deal in its divided Parliament.
The EU’s decision also means there is time for the U.K. to hold an election this year without the imminent prospect of a departure from the bloc without a ratified deal—an outcome that many British lawmakers believe could cause serious economic disruption.
Mr. Johnson has called for a ballot on Dec. 12, but as his government is in a minority in Parliament there is no certainty that he will have enough support to hold one.
British lawmakers are expected to vote later Monday on Mr. Johnson’s proposal to hold an election. The proposal would need the support of two-thirds of the House of Commons, meaning the main opposition Labour Party would need to back it. But Labour officials have indicated the party would abstain from the vote.
If that route fails, there is another pathway to an election this year—one that requires only a simple majority in Parliament but could potentially be derailed by amendments.
Two anti-Brexit parties—the Liberal Democrats and the Scottish National Party—have said they would support an alternative route for an election on Dec. 9. Their hope is that a new Parliament would be more inclined to back a second Brexit referendum.
U.K. government officials are seriously considering the offer from the Liberal Democrats and SNP. A one-line bill calling for an election could be introduced to Parliament as early as Tuesday if Labour doesn’t back fresh elections on Monday, one official said. A Liberal Democrat official said the party believes that route—which would require only a simple majority to pass—is the clearest to a ballot.
Mr. Tusk said on Twitter that the EU must still officially sign off the Brexit extension agreement through a special written procedure that avoids EU leaders having to meet for a summit. An EU official said that is due to take place by Wednesday at the latest.
Under the draft extension text, the EU said that it won’t engage in further negotiations on the agreement reached earlier this month with Mr. Johnson.
The U.K. requested the extension after lawmakers passed a bill that forced the government to ask for one and last week rejected Mr. Johnson’s speedy timetable for debating and ratifying the withdrawal deal. Mr. Johnson had previously said there were no circumstances under which he would extend Brexit beyond Oct. 31.
Though the deal has been approved in principle by British lawmakers, they have yet to examine it in detail—and any approval might come with strings attached.
The EU agreement to grant an extension had to be unanimous. French officials had been reluctant to agree to a three-month Brexit extension, with French President Emmanuel Macron saying the U.K.’s departure needed to be completed as soon as possible to allow the bloc to tackle other issues. Mr. Macron spoke over the weekend with Mr. Johnson.