Opening five days of debate on the Brexit deal, May told Parliament that the British people had voted in 2016 to leave the European Union, and it was the "duty of this Parliament to deliver on the result" of the referendum.
Theresa May has been forced to publish the full legal advice on Brexit after MPs found her ministers in contempt of Parliament.
Shadow Brexit Secretary Sir Keir Starmer said Tuesday's vote had "huge constitutional and political significance".
Anti-Brexit campaigners protest in Westminster in London.
"There can be no question that he or the government has acted in a manner which is contemptuous of this house", she said.
The vote has little direct impact on the Brexit debate, but reflects mounting tension between the government and Parliament over the next steps in Brexit.
The European Court of Justice's senior legal adviser has indicated that the United Kingdom could simply change its mind and abandon Brexit, without needing the approval of the 27 other EU states.
But her chances of winning majority backing for the deal look slim.
Labour, the Northern Irish DUP, SNP and Lib Dems, the Greens and Plaid Cymru had triggered the "contempt" action after Attorney General Geoffrey Cox failed to comply with MPs' demands.
If, against the odds, May wins the vote, Britain will leave the European Union on March 29 on terms negotiated with Brussels - its biggest shift in trade and foreign policy for more than 40 years.
Reflecting on her personal journey, May added: "I have spent almost two years negotiating this deal".
"The Government must not be allowed to use this chaotic situation to take focus away from the mess they are making of Brexit".
Former attorney general Dominic Grieve's amendment will make any such statement itself amendable, meaning that MPs can effectively direct the Government on how to respond to the defeat of Mrs May's plans.
"If I had banged the table, walked out of the room and at the end of the process delivered the very same deal that is before us today some might say I'd done a better job".
The formal advice from a European Court of Justice advocate general - not binding but usually heeded by the court - suggested to some lawmakers that revoking the "Article 50" divorce notice was an option.
He argued that it would not be "in the national interest" to publish his advice in full as it would break a longstanding convention that law officers' advice to ministers is confidential.
"We as 27 have a clear position on fair competition, on fish, and on the subject of the EU's regulatory autonomy, and that forms part of our position for the future relationship talks", said Macron.
But the advice bolstered anti-Brexit campaigners, who hope the decision to leave can be reversed.
Downing Street reaffirmed on Tuesday that May has absolutely no intention of doing so - and that the ruling was in either case only advisory.
With just seven days left to try to turn the overwhelming opposition for her Brexit deal around, the prime minister deployed her attorney general - an ardent Eurosceptic - to the dispatch box to try to sell her deal.
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