British Prime Minister Theresa May has delayed a crucial vote in the House of Commons — originally scheduled for tomorrow — on her controversial Brexit deal, acknowledging that she would have lost the vote by "a significant margin."

Why it matters: An embarrassing defeat may have dealt the final blow to May's premiership, with the opposition Labour Party saying earlier this month that it would push for a no-confidence vote against May if her Brexit deal failed. It's not clear what she can achieve by reopening talks with Brussels, as European Union leaders have repeatedly said the current negotiated deal is the only one on the table.

The big picture: The delay is the latest in a string of Brexit headaches for May's government, beginning with an assessment by both the U.K. Treasury and the Bank of England that leaving the EU would significantly damage the British economy. May's government was then found in contempt of Parliament for the first time in history for refusing to publish its full legal analysis on Brexit.

What's next: May told the Commons that she will return to Brussels for an EU summit Thursday, where she will seek to improve the backstop provision in her deal — which is designed to prevent a hard border between the Republic of Ireland and Northern Ireland. But European Council President Donald Tusk tweeted Monday that the EU "will not renegotiate the deal, including the backstop," so it's unlikely anything significant comes of this week's summit.

The European Court of Justice also ruled Monday that the U.K. could unilaterally cancel Brexit without the permission of the other 27 EU states. May has made clear, however, that this will not happen as long as she is in power.

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