Boris Johnson called European Council President Donald Tusk to confirm that he will send the extension letter this evening, an EU official said.
Mr Tusk will start to consult EU leaders on how to react which could “take a few days,” they added.
But the Prime Minister also wrote a letter to MPs saying he would tell the EU that “further delay is not the solution”.
It comes after MPs voted to withhold approval of the Brexit plan until the legislation needed to implement it has been enacted by Parliament.
Mr Johnson is bound by law under the Benn Act to send a letter to the EU requesting an extension to the the October 31 Brexit deadline after the deal was not been ratified by Parliament before October 19.
The European Council President earlier tweeted that he was “waiting for the letter” from Boris Johnson.
Mr Tusk said: “Waiting for the letter.
“I just talked to PM Boris Johnson about the situation after the vote in the House of Commons.”
In a letter to MPs today, Mr Johnson that “further delay is not a solution”.
He added: “The public want us to get Brexit done so the country can move on.
“The best thing for the United Kingdom and the European Union is for us to leave with this deal on 31 October”.
.Georgina Wright, a senior researcher from the Institute for Government think tank reckons the EU has four options.
The first would be to reject any Brexit extension that the UK asks for, in a bid to “focus minds in Westminster” with a no-deal exit on October 31.
The second option would be to wait and see what Parliament does next and then make a decision.
Thirdly, the EU 27 could offer a short extension to mid-November to allow for all the legislation to be voted on. A no deal could still happen if this deadline is missed.
And finally, the EU could offer a long delay – possibly into 2020. This could be done if MPs indicate they want to hold a second referendum
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