He said giving Parliament power to direct the government's hand in talks would be "an unconstitutional shift which risks undermining our negotiations with the European Union".
In a day of drama, May's position seemed suddenly weaker when junior justice minister Phillip Lee, who has always been critical of the government's Brexit strategy, resigned and said he would vote against the government.
Remain-supporting Conservative MPs had threatened to defeat the government on an amendment to the bill which would have given Parliament a wide-ranging veto to May's Brexit deal, or even force a second referendum.
Dismissing the Government's compromise, she tweeted: "Merely issuing a statement in response would make it a meaningless final vote".
Her fellow Conservative backbencher Stephen Hammond said: "Parliament must be able to have its say in a "no deal" situation".
Theresa May ultimately persuaded all but two of her MPs to back her in the decisive vote in Westminster on Tuesday - but she increasingly appears little more than a hostage to the warring factions in a bitterly divided Conservative party.
Further clashes were expected on Wednesday during debate on amendments relating to how closely Britain stays aligned with the EU's economy after leaving.
Two days of debate on the laws that will end Britain's European Union membership have crystallised long-running divisions within May's party about the best strategy for leaving the European Union, bringing to a head issues that will determine the relationship between the world's fifth-largest economy and its biggest trading bloc.
"We will wait and see the details of this concession and will hold ministers to account to ensure it lives up to the promises they have made to Parliament".
The EU published an explainer of its own backstop proposal in a bid to convince the United Kingdom government of its merits, but Theresa May has said it is not acceptable because it creates a regulatory border between Northern Ireland and the UK.
Brexit protesters outside Parliament House.
Parliamentary debates about complex legal amendments rarely rouse much heat, but passions run high over anything to do with Brexit. The Daily Express thundered: "Ignore the will of the people at your peril".
The Sun had a frontpage of British icons including Stonehenge, a fish and chip shop, a London bus and a football, saying "Great Britain or Great Betrayal".
The main point of contention between those who want to keep the closest possible ties with the European Union and those who aim for a clean break is a demand to give parliament a "meaningful vote" on any agreement May negotiates with Brussels.
The frontpages of Leave-backing British newspapers said accepting the amendments would betray the 52% who backed Brexit in the seismic 2016 referendum.
Both the U.K.'s and EU's proposals are supposed to be temporary and to act as an insurance policy until the two sides strike a long-term trade deal that maintains an invisible border. "They want us to regain control of our borders", he said.
"Parliament, don't stand against the people - implement their will!"