Despite strong opposition in her Conservative party and criticism in Brussels, May has stuck by the so-called Chequers proposal to keep close trade ties with the European Union after Brexit on March 29 next year, AFP reported.
"The alternative to that will be not having a deal," she told the BBC in an interview out Monday in response to the opponents of her blueprint for Brexit.
May will meet EU leaders in Salzburg on Wednesday and Thursday, as she seeks a breakthrough in talks on the Brexit divorce and the future UK-EU trading relationship.
The International Monetary Fund on Monday said Britain's economy would suffer "substantial costs" should it leave the EU without a deal.
Brussels and London have failed to resolve "fundamental" aspects of Brexit and this could leave London defaulting to World Trade Organization (WTO) tariffs, the IMF said in its annual outlook on the UK economy.
"Fundamental questions -- such as the future economic relationship between the two and the closely-related question of the status of the land border with Ireland -- remain unanswered," it noted in a statement.
"Resolving these questions is critical to avoid a 'no-deal' Brexit on WTO terms that would entail substantial costs for the UK economy – and to a lesser extent the EU economies – particularly if it were to occur in a disorderly fashion," the IMF added.
Problems in Parliament
The gloomy assessment contrasts with that of May, who indicated last week that a no-deal hard Brexit would not be a disaster for Britain.
May remains confident of striking an acceptable deal with Brussels.
But even if she gets an accord in the coming weeks, it must be signed off in Parliament, where she can only muster a slender majority.
The main opposition Labour party's Brexit spokesman said Sunday that it could not back a deal unless it delivered the "exact same benefits" as Britain currently has inside the single market and customs union – an unlikely prospect.
That would mean only a small number of May's Conservative MPs need to rebel in order to bring down her blueprint – and plenty of hardcore Brexiteers are infuriated by it.
May expressed confidence parliament would approve the deal -- but warned there was no alternative if Britain wanted to avoid a "no deal" scenario.
"Do we really think... that if parliament was to say, 'No, go back and get a better one', do we really think the EU is going to give a better deal at that point?" she said.
May has proposed that Britain follow EU rules in trade in goods after Brexit, to protect manufacturing supply lines and avoid a "hard border" between Northern Ireland, part of the United Kingdom, and the Republic of Ireland.
May also insisted no other plan on the table would ensure "frictionless" trade on the island of Ireland.
But critics say her proposal would tie Britain too closely to the EU, and argue that the Irish issue can be resolved through trusted trader schemes and the use of technology.
Former foreign minister Boris Johnson, who quit in July in protest at the Chequers plan, launched a fresh attack on it in his weekly newspaper column on Monday.
"The whole thing is a constitutional abomination," Johnson, who has previously compared the plan to a "suicide vest", wrote in The Daily Telegraph.