It leaves May facing an nearly certainly losing battle to overturn the 230 majority defeat of her deal in tonight's meaningful vote.
"She said she had been advised this letter would have legal force in worldwide law".
All eyes will then be on another Commons vote, expected by Thursday, in favour of delaying the March 29 deadline.
However, he said the situation was unchanged if a deal that supersedes the backstop can not be reached "through no such demonstrable failure of either party".
Mark Francois, vice chairman of the ERG, said he was "wholly unconvinced" by the deal and added that many of his colleagues felt the same way.
What happens after that is anyone's guess, with many MPs, including the Labour party, advocating for a second referendum.
"We believe this deal has a chance".More news: Reputed Gambino crime boss shot and killed outside home in NY
This option is likely to prove popular, since politicians on both sides of the Brexit debate fear time is running out to secure an orderly withdrawal by March 29.
The pound was last trading 0.6 percent higher on the day at $1.3223, having been as low as $1.2945 at one stage on Monday.
The EU will have to agree to any extension to article 50 and the commission's secretary general, Martin Selmayr, told ambassadors on Monday that the safest delay to Brexit would only be up to 23 May, ensuring that elections to the European parliament would not create complications.
Prime Minister Theresa May rushed to Strasbourg on Monday evening to seek concessions from the European Union in a last-ditch attempt to avoid another humiliating defeat in Parliament of her deal to exit the bloc.
May's conservative party has 314 members, who are deeply divided over May's deal.
Critics argue that giving MPs less than 24 hours to make their decision is inadequate, nevertheless, May is hoping that parliament will rally behind her "improved" deal and put an end the Brexit chaos, at least for now.
The EU said there would be no more negotiations with London on the divorce terms, struck with May after two-and-a-half years of tortuous negotiations.More news: Paul Manafort now faces 7 years in prison, fresh NY charges
Theresa May strikes an agreement with Jean-Claude Juncker. "There will be no further interpretations of the interpretations, no further assurances of the reassurances if the "meaningful vote" tomorrow fails".
Whatever Parliament decides this week, it won't end Britain's Brexit crisis.
A spokesman for European Council President Donald Tusk, representing EU governments, said Britain would have to provide a "credible justification" for any request for a delay.
Britons voted by 52-48 percent in 2016 to leave the European Union but the decision has not only divided the main parties but also exposed deep rifts in British society, bringing concerns about immigration and globalisation to the fore.
Brexit will pitch the world's 5th largest economy into the unknown and many fear it will divide the West as it grapples with both the unconventional US presidency of Donald Trump and growing assertiveness from Russian Federation and China.
"When the deal was originally defeated in January, sterling traded down at $1.26".More news: Cristiano Ronaldo celebrates with Simeone's 'big balls' gesture