Some opponents are calling for a second referendum now that the costs of leaving the European Union have become clear, but May says that would violate the trust of the 17.4 million people who voted for Brexit in 2016.
The G20, which this year holds its summit in Buenos Aires, is made up of the 19 of the world's most industrialised nations, plus the European Union.
Ms. May visited Scotland earlier to garner support for the Brexit deal, which she says, is good for all parts of the United Kingdom.
"If they can not look their constituents in the eye and say it is, they must vote this deal down and then hand the decision back to the people".
MPs will vote on Prime Minister Theresa May's Brexit deal in the Commons in two weeks, but with staunch cross-party opposition persisting, the PM may well see her deal voted down.
She added: "I think what we have seen revealed in numerous Labour Party's comments is that what they actually want is a general election, and that means they are not acting in the national interest, they are putting their narrow party interests first".
The Conservative MP, who campaigned for Remain, labelled negotiations over Galileo, the EU's strategic Satellite Navigation system, "a foretaste of what's to come under the government's Brexit deal".
The people she most needs to persuade of the merits of her deal are back in the United Kingdom: the 650 members of the House of Commons who on December 11 will vote on whether to give it their support.
So if May wants to avoid losing both Brexit deal and her job, it may be that she needs to stop being quite so coy about the path or paths she may take, if (when?) she loses in 10 days.
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The Labour party and numerous Prime Minister's own MPs have also signalled their oppposition to the agreement.
May has said if lawmakers in parliament's House of Commons reject the agreement it could see the world's fifth-largest economy leaving the bloc without a deal, or not leaving at all.
He added: "I think that the reactions to some of the Treasury analysis have been completely overblown". "It says we will be able to do those trade deals, and we will be able to do them with the U.S. and others".
The Prime Minister was speaking from nearly 7,000 miles away in Buenos Aires at the G20 with Donald Trump and Vladimir Putin.
It also comes as the Conservatives and Labour, divided over Brexit, also find themselves unable to come to an agreement over the hotly anticipated televised debate on Brexit.
The motion will be signed by the pro-independence Scottish National Party and Scottish Greens and the anti-independence Labour and Liberal Democrats.
The decision to back a People's Vote was taken at a meeting of council leaders in Edinburgh today.
He added: "It is a deal in name only".
Writing in the Daily Telegraph, arch-Brexiteer Jacob Rees-Mogg accused the Government of an effort to "frighten and to gull (people) into acquiescing to a non-Brexit Brexit".