London (CNN) UK Prime Minister Boris Johnson has been forced to ask the European Union for an extension to the Brexit process, an outcome he once notoriously described as worse than being "dead in a ditch."

But in an extraordinary development that followed a bruising defeat in Parliament, he simultaneously disowned the request, declaring that the three-month delay would be "corrosive."

Johnson was compelled by law to demand the delay after lawmakers withheld approval of his Brexit deal. In a day of intense political drama in London, MPs voted to postpone ratification of the deal until Parliament has passed the complex set of legislation required to enact it.

Downing Street demonstrated Johnson's disdain for the process by sending an unsigned photocopy of the legally mandated letter by email to the EU Council President, Donald Tusk. It was accompanied by a covering letter from a senior civil servant explaining that the letter was being sent in order to comply with a law passed by Parliament last month which is designed to prevent a no-deal Brexit.

Boris Johnson: I will not negotiate a delay with the EU

Boris Johnson: I will not negotiate a delay with the EU 01:55

Boris Johnson: I will not negotiate a delay with the EU

A third letter was sent to EU leaders, in which Johnson argued that any further delay would "damage the interests of the UK and our EU partners, and the relationship between us." Tusk said he had received the correspondence and would consult with other EU leaders on next steps.

Johnson's disavowal of the extension request is likely to be challenged in court. The government also said it would attempt to stage another vote on the deal on Monday.

"I will not negotiate a delay with the EU, and neither does the law compel me to do so," Johnson had told lawmakers earlier. "Further delay will be bad for this country."

The Labour leader, Jeremy Corbyn, blasted Johnson from across the floor of the House of Commons. "The Prime Minister must now comply with the law," he said. "He can no longer use the threat of a no-deal crash-out to blackmail MPs to support his sellout deal."

As lawmakers were debating in Parliament, thousands of protesters marched through central London to demand a second Brexit referendum -- an eventuality that now seems a lot less improbable in the light of Saturday's developments.

Photos: In photos: Thousands march for second Brexit referendum Crowds gather during a People's Vote campaign march in London, England, on Saturday, October 19. Hide Caption 1 of 10 Photos: In photos: Thousands march for second Brexit referendum Crowds march through central London holding European Union flags and anti-Brexit signs. Hide Caption 2 of 10 Photos: In photos: Thousands march for second Brexit referendum Pro-Brexit and anti-Brexit demonstrators argue at Parliament Square as lawmakers in the House of Commons debate nearby. Hide Caption 3 of 10 Photos: In photos: Thousands march for second Brexit referendum Anti-Brexit buttons rest on a table covered with an EU flag in central London. Hide Caption 4 of 10 Photos: In photos: Thousands march for second Brexit referendum An aerial view of the protesters in Parliament Square in London. There are no official numbers for the size of the turnout. Hide Caption 5 of 10 Photos: In photos: Thousands march for second Brexit referendum Actors Paul McGann and Patrick Stewart join other protesters demanding a second vote. Hide Caption 6 of 10 Photos: In photos: Thousands march for second Brexit referendum Michela Beltracchi, an Italian who has lived in the UK for over 10 years, protests as part of the rally. Hide Caption 7 of 10 Photos: In photos: Thousands march for second Brexit referendum Protesters celebrate the passing of the Letwin amendment in the House of Commons. Hide Caption 8 of 10 Photos: In photos: Thousands march for second Brexit referendum A group of dogs dressed in European Union flag sweaters are seen at the anti-Brexit march in Parliament Square. Hide Caption 9 of 10 Photos: In photos: Thousands march for second Brexit referendum Mayor of London Sadiq Khan, who supports a second referendum, gives a media interview as demonstrators gather on Park Lane. Hide Caption 10 of 10

Moment of victory denied

Government aides were furious at the result of the vote in Parliament, as it denied Johnson the chance to declare a Brexit victory on Saturday. After two days of arm-twisting since Johnson returned with a new deal from Brussels on Thursday, Downing Street believed it had secured the numbers required to pass it, albeit by a razor-thin margin.

Johnson's nemesis was former Conservative government minister Oliver Letwin, expelled from the party when he voted for the law that prevents a no-deal Brexit. Proposing the measure that delayed Parliament's approval, Letwin said it was an "insurance policy" to ensure the UK would not "crash out" of the European Union without a deal on October 31.

Under legislation known as the Benn Act, the UK government was required to request an extension to the Brexit process until January 31, if a deal was was not ratified by the end of Saturday.

Those provisions would have fallen away if Johnson had succeeded in getting his deal through the House of Commons. But Letwin and his allies were concerned that a no-deal Brexit could still happen at the end of October if, by then, lawmakers had failed to pass the complex set of legislation required to enact the departure deal.

In the end, the Letwin amendment passed by 322 votes to 306. Not even the backing of former Prime Minister Theresa May, who spoke in favor of her successor's deal despite having once rejected a not dissimilar proposal from the EU, was enough to prevent the defeat.

Former Prime Minister Theresa May spoke in favor of Johnson's deal.

In a twist of political fate, the outcome was decided by the 10 MPs of the Democratic Unionist Party, the Northern Ireland group that nominally props up Johnson's minority government in the House of Commons.

The DUP was deeply unhappy with terms of Johnson's deal, under which Northern Ireland effectively remains in the EU customs union, separating the province economically from the rest of the UK. Its MPs were particularly furious that Johnson had cast them aside, traveling to Brussels on Thursday to sign his Brexit deal with EU leaders without securing their support.

"We are cut off from the country to which we belong," said Sammy Wilson, the DUP's Brexit spokesman, in an impassioned speech in the House of Commons.

What happens next?

Johnson's contradictory correspondence to the EU is bound to land him in front of a judge. A related case was already due to be heard at Scotland's highest court, the Court of Session in Edinburgh, on Monday, and this will likely be the venue for the first legal challenge.

Joanna Cherry, the Scottish National Party lawmaker who has led many of the legal challenges over Brexit, confirmed the hearing would go ahead.

An aerial view of the anti-Brexit protest in Parliament Square, London.

Opposition lawmakers were furious at the developments. John McDonnell, Labour's finance spokesman, said on Twitter: "Johnson is a Prime Minister who is now treating Parliament and the Courts with contempt. His juvenile refusal to even sign the letter confirms what we always suspected that Johnson with his arrogant sense of entitlement considers he is above the law and above accountability."

Meanwhile Jacob Rees-Mogg, the leader of the House of Commons, said the government would bring forward another vote on Johnson's Brexit deal on Monday.

Ordinarily, the same provision can't be voted on twice in a parliamentary session. That convention thwarted ex-Prime Minister May's plans to hold repeated votes on her withdrawal deal. But it was not clear whether Saturday's proceedings in Parliament counted as a vote on Johnson's deal, because it was amended by the Letwin measure. Speaker of the House John Bercow said he would rule on the matter Monday