In his own signed letter to Tusk, Johnson said he was confident that the process of getting the legislation through Britain's parliament would be completed before October 31.
Mr. Letwin, who supports Mr. Johnson's Brexit deal, argued that the amendment was simply a safety net to prevent pro-Brexit hard-liners from sabotaging the implementing legislation and, in the ensuing political vacuum before the October 31 deadline, engineering the no-deal rupture that some want.
The Downing Road supply stated Johnson had despatched a photocopy of the letter contained within the legislation that requires him to ask for the delay if there isn't any Brexit deal, however didn't signal it.
Johnson's defeat in the British parliament over the sequencing of the ratification of his deal exposed the prime minister to a law passed by those opposed to a no deal departure, demanding he request a delay until January 31.
Saturday's amendment, put forward by former Conservative cabinet minister Oliver Letwin, deflated Johnson's big Brexit day just as hundreds of thousands gathered to march on Parliament demanding another referendum on European Union membership.
Johnson was obliged to file the request by law if he failed to gain parliamentary approval for the new Brexit deal by Saturday.
"I will tell our friends and colleagues in the European Union exactly what I have told everyone in the last 88 days I have served as Prime Minister; that further delay will be bad for this country, bad for the European Union and bad for democracy".
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Oliver Letwin, the lawmaker behind Saturday's amendment, said on Sunday that he believed Johnson could probably get his Brexit deal over the line.
Protesters outside parliament cheered as legislators backed Letwin's amendment. The amendment effectively builds into the law another insurance policy to avoid a "no-deal" Brexit.
EU ambassadors agreed on Sunday morning that the withdrawal agreement would be sent to the European parliament on Monday.
Labour's Brexit spokesman Keir Starmer said the party would put forward amendments to Johnson's Brexit legislation, particularly aimed at closing the "trap door" to a no-deal Brexit kicking in at the end of a transition period in December 2020.
The unseemly missives to Brussels - called "silly" by critics - were prompted by the law enacted in September by Conservative rebels and opposition MPs to force the government to seek the extension if an agreement is not passed by the House of Commons by 11 pm on October 19.
To make up for the votes of 10 DUP lawmakers, Johnson has tried to persuade members of the left-of-centre Labour Party to support the deal.
The decision is a huge setback to Johnson, who defiantly told MPs immediately after the vote that he was "not daunted or dismayed" by the result and that he believes he can still command "overwhelming" support for the new European Union divorce plan. "That is what we are going to do next week", he told the BBC.
Tusk confirmed he has received the extension request from Johnson.
He says lawmakers face the option of either approving the deal or propelling the United Kingdom to a disorderly no-deal exit that could divide the West, hurt global growth and bring renewed violence to Northern Ireland.