After losing the crunch vote in a historic Saturday session in the House of Commons, the Prime Minister ordered a senior diplomat to send an unsigned photocopy of the request for a delay, which was forced on him by MPs last month, the BBC reported.
The Prime Minister was required by the law called the Benn Act to ask the EU for an extension to the October 31 deadline in case he lost the Commons vote.
The request was accompanied by a second letter, signed by Johnson, which made clear that he personally believed a delay would be damaging.
It said the government will press on with efforts to pass the revised Brexit deal agreed with EU leaders last week into law, and that he was confident of doing so by October 31.
In it, the Prime Minister said any further hold-up would be "deeply corrosive", and would "damage the interests of the UK and our EU partners".
He added that the UK would continue to ratify the deal and urged Brussels to do the same.
A third cover letter written by Tim Barrow, the UK's Permanent Representative in Brussels, explained the first letter complied with the Act as agreed by Parliament.
Tusk tweeted that he had received the extension request and would consult EU leaders "on how to react".
This development comes after the MPs, at the first Saturday sitting in the Commons since the 1982 Falklands War, voted by 322 to 306 in favour of an amendment withholding the approval of Johnson's Brexit deal until all the necessary legislation was completed, the BBC reported.
Tabled by Conservative MP Oliver Letwin, the amendment was intended to ensure that Johnson would comply with the terms of the Benn Act.
Under that Act, passed by MPs last month and which required the Prime Minisrer to seek a Brexit extension, Johnson had until 11 p.m. on Saturday to send a letter requesting a delay.
The fate of Johnson's deal now lies in the hands of Speaker John Bercow - who on Saturday hinted that he might not allow a meaningful vote on it - the rebel MPs and other EU leaders, especially French President Emmanuel Macron and German Chancellor Angela Merkel, The Daily Mail said in a report on Sunday.
Downing Street is hopeful that other EU leaders will refuse to allow an extension.
A source said that they put the chances at "about 50 per cent", adding: "Macron has been particularly trenchant in private about not wanting to extend, and we hope he could take Merkel with him."
On Saturday night, the French President's office signalled that they would not back an extension, which officials said was "in nobody's interest".
( With inputs from IANS )