"I believe that the deal we have negotiated is a good deal", she said, adding: "I'm continuing to listen to colleagues on that and considering a way forward".
Opening the second of five days of debate, Home Secretary Sajid Javid told legislators Wednesday they should back the Brexit agreement to safeguard Britain's vital security relationship with the EU.
Some lawmakers have called on May to change the deal, and have suggested she could use an European Union summit next week to try to win some concessions from officials to try to ease some of their concerns.
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"It is a vote in which the future of their country is at stake", he said.
But many members of Parliament worry that the transition period is a bridge to nowhere.
Last night, the chairman of the influential 1922 Committee of Conservative backbench MPs publicly asked the Prime Minister to call off Tuesday's vote last night.
Tory resistance to the Brexit plan could be bolstered by the knowledge that voting down the deal will not necessarily result in the collapse of the Government given the DUP's stance.
In the document, attorney general Geoffrey Cox warned that Britain would need European Union permission to leave the backstop, keeping Britain in "protracted and repeating rounds of negotiation". But the defeat demonstrated the fragility of May's government, which does not have a majority in Parliament.
"I think members on both sides know she has worked phenomenally hard to try to secure the best agreement", he said.More news: Ballon d'Or: Modric wants to retire at Real Madrid
It is a key condition of the continued support of the Northern Irish Democratic Unionist Party, which is propping up May's minority government.
"This was not about doing deals, it was about listening", said one leading pro-Brexit lawmaker.
Deputy DUP leader Nigel Dodds said: "The overall context of this is ... a deeply unattractive, unsatisfactory presentation and he (Cox) needs to therefore, rather than recommend this agreement, recommend that it is rejected".
"There are those who want to frustrate Brexit and overturn the vote of the British people", Ms.
The Newbury MP voted in favour of an amendment which will hand power to MPs if Theresa May's Brexit withdrawal agreement is rejected.
In one potential concession, May said she recognised that there were concerns among lawmakers about the so-called Northern Irish backstop and she was looking at whether parliament could be given a greater role in deciding whether to trigger it.
Another fellow Cabinet minister, worldwide trade secretary Liam Fox, also backed the deal, warning there was a "natural "Remain" majority" in Parliament and any attempt to overturn the 2016 referendum vote in favour of Brexit would be a "democratic affront".
May faced the prospect of "hitting a brick wall at speed" and predicted that no consensus would found in the House of Commons on Brexit.
"I am not however prepared to give Theresa May a blank cheque and am therefore only prepared to support a deal that will deliver the "exact same benefits" as the United Kingdom now has as a member of the single market and customs union and ensures a fair management of migration in the interests of the economy and communities".