London -- British Prime Minister Theresa May's proposal for how the United Kingdom should extract itself from the European Union appeared on Tuesday to be facing another defeat in Parliament. Lawmakers were to cast votes Tuesday night, for the second time, for or against the proposed Brexit deal that May has spent more than two years negotiating with the EU.

She won some last minute concessions from EU negotiators on Monday to try and get a majority of the House of Commons on board with her plan, but her own attorney general made it clear the changes were effectively cosmetic, and would not give the U.K. the legal power to cut loose of the EU unilaterally if negotiations drag on. It appeared to doom May's plan, as pro-Brexit lawmakers have remained adamant that any Brexit deal must not leave Britain trapped under EU rules post-breakup.

Attorney general Geoffrey Cox said the changes May won from Europe did "reduce the risk" that Britain could be stuck inside EU regulations indefinitely, but did not eliminate it.

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The so-called "meaningful vote" in the House of Commons on Tuesday evening comes less than three weeks before Britain is due to leave the EU on March 29.

Lawmakers defeated May's deal by a whopping 230 votes in January, but May hoped the changes she secured from the bloc would be enough to persuade many to change their minds. That appeared unlikely after Cox's assessment, however.

What happens after the vote

May herself has warned that another vote against her plan would dramatically increase the chances of a potentially catastrophic "no-deal" Brexit -- an exit without any agreement in place with Europe -- or a delayed Brexit, or possibly even no Brexit at all.

EU leaders have clearly warned the Brits that negotiations will not be resumed if London rejects May's deal again.

"In politics, sometimes you get a second chance. It is what you do with this second chance that counts. Because there will be no third chance," European Commission President Jean-Claude Juncker said Tuesday. "Let's be crystal clear about the choice: it is this deal or Brexit might not happen at all."

If the House of Commons rejects May's deal again, they will reconvene on Wednesday for a new vote on whether Britain should leave the EU without an agreement. Very few members of Parliament have said they'd be willing to risk a no-deal Brexit, and it's expected they would formally reject the prospect in a vote.

If they do, there will be another vote, expected on Thursday, when the lawmakers would vote on whether to ask the EU to delay the actual Brexit date.

Britain's Prime Minister Theresa May speaks in Parliament in London, March 12, 2019, in this screen grab taken from video. REUTERS

May, her voice weak and raspy as she addressed Parliament ahead of the vote on Tuesday, warned that request to delay Brexit "would merely pass control to the European Union." She said Europe alone would be able to dictate the duration of any delay, and that it could "even lead to a second referendum, with all the damage that would do to our democracy."

If that happened, May said "it would be no good blaming the European Union" for the British Parliament's "failure to come together in the national interest and deliver."

May's fellow Conservative parliamentarian Bob Seely underscored Mrs. May's warning in a tweet, saying: "Today is our Hotel California moment. If we don't check out tonight, we may never leave."