What happens next: A general election is "not unlikely at all," while a second referendum is "an option for the future," not today.
A 2016 referendum to pull Britain out of the EU won narrowly. Some in the Labour Party have called for a second referendum to give the public final say on the Brexit deal.
The country is set to leave the EU on March 29.
Earlier May warned that a leadership change wouldn't make Brexit negotiations easier.
Opponents in her Conservative Party have threatened to unseat her and the former Brexit secretary suggested she failed to stand up to bullying from European Union officials.
As furious Conservative rebels try to gather the numbers to trigger a no-confidence vote, May insisted she hadn't considered quitting.
"A change of leadership at this point isn't going to make the negotiations any easier and it isn't going to change the parliamentary arithmetic," she told Sky News.
May added that the next seven days "are going to be critical" for successful Brexit talks, and that she will be travelling to Brussels to meet EU leaders before an emergency European Council summit on November 25.
An announcement last week that Britain has struck a draft divorce agreement with the EU triggered a political crisis in Britain, with the deal roundly savaged by both the opposition and large chunks of May's own Conservatives. Two Cabinet ministers and several junior government members quit, and more than 20 MPs have submitted letters of no confidence in May.
Forty-eight such letters — or 15 per cent of Conservative MPs — are needed for a leadership challenge vote.
Asked about the attacks directed at her, May said: "It doesn't distract me. Politics is a tough business and I've been in it for a long time."
Dominic Raab, who quit as Brexit Secretary, said "there is one thing missing and that is political will and resolve".
He told the Sunday Times: "If we cannot close this deal on reasonable terms, we need to be very honest with the country that we will not be bribed and blackmailed or bullied and we will walk away".
Many pro-Brexit Conservatives want a clean break with the EU and argue that the close trade ties between the UK and the EU called for in the deal would leave Britain a vassal state, with no way to independently disentangle itself from the bloc.
The draft agreement envisions Britain leaving the EU as planned on March 29, but remaining inside the bloc's single market and bound by its rules until the end of December 2020.
It also commits the two sides to the contentious "backstop" solution, which would keep the UK in a customs arrangement with the EU until a permanent trade treaty is worked out.
That will serve to guarantee that the border between the UK's Northern Ireland and EU member Ireland remained free of customs checkpoints after Brexit.
Both Britain and the EU want to ensure Northern Ireland's hard-won peace process isn't undermined, but reaching an agreement on how to achieve that had long been a key obstacle in the negotiations.
May's Conservatives don't have a parliamentary majority, and it's not clear if her deal can successfully pass Parliament.
Raab said that while the agreement was "fatally flawed," it's not too late to change that.
"I still think a deal could be done but it is very late in the day now and we need to change course," he told the BBC.
- AP, Bloomberg
