British lawmakers are due to vote Monday afternoon on Mr Johnson's call for an early election to be held on Dec 12, while his accord with Brussels would be put before parliament for approval by early next month.
"The decision is expected to be formalized through a written procedure", he said, after ambassadors met in Brussels to approve the extension.
EU Council President Donald Tusk said it was a "flextension" - meaning the United Kingdom could leave before the deadline if a deal was approved by Parliament.
Though most parties in parliament support an election, many in the opposition, particularly the Labour Party, are wary that it could be used to crash the United Kingdom out of the EU.
French President Emmanuel Macron blocked the EU's attempt to delay Brexit for three months, insisting instead on a one-month delay to November 30. In previous snap elections - called earlier than the scheduled date - they have also shown a tendency to punish parties in power.
12 December has emerged as Boris Johnson's favoured date.
"We hope this to be concluded by Tuesday or Wednesday", the official said.
I think it is quite obvious that no one thought that the whole Brexit process would be so hard and there would have been so many issues to take into account from Northern Ireland to tarrifs and taxes.
The bloc's leaders have agreed in principle to a delay but have yet to decide for how long.
However a supply at Johnson's Downing Road workplace mentioned the prime minister would do all he can to pressure an election to "get Brexit done", together with contemplating choices provided by different opposition events.
She also repeated the party's position that it will back an early election only after Johnson explicitly says there won't be a no-deal Brexit.
The EU has agreed there must be an extension however has set no departure date, saying it could wait to see how parliament voted on Johnson's name for an early election.
The decision came following a 30-minute meeting of the 27 European Union ambassadors in Brussels after France dropped its objections that blocked the decision last week.
The extension is through to January 31, giving the British parliament plenty of time to debate British Prime Minister Boris Johnson's Brexit deal.
Johnson insisted on Friday that British lawmakers need to commit to the December 12 date for the election "to have any credibility about delivering Brexit".
MPs will vote on Monday on whether to support the Prime Minister's election plans.
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