The U.K. pound plunged past an 20-month low Monday after Prime Minister Theresa May moved to delay a key parliamentary vote on her Brexit deal that looked certain to be rejected by lawmakers, throwing both her government and the broader plans to leave the European Union in chaos.

The Prime Minister said she would delay the vote, which was slated to begin late Tuesday, as she seeks to find support in her fractured party for the so-called EU withdrawl agreement, a deal that sets out Britain's Brexit terms but leaves open for negotiation its future trading relationship with the bloc.

"If we went ahead and held the vote tomorrow the deal would be rejected by a significant margin," May told lawmakers. "We will therefore defer the vote scheduled for tomorrow and not proceed to divide the House at this time."

The pound was marked 1.7% lower against the U.S. dollar on the session and changing hands at 1.2511, the lowest since April 13, 2017.

The delay came just hours after her official spokesperson said the vote would definitely happen, and Europe's highest court confirmed an earlier opinion that Britain can, if it wants, unilaterally cancel Brexit proceedings and remain a member of the European Union.

"Such a revocation, decided in accordance with its own national constitutional requirements, would have the effect that the United Kingdom remains in the EU under terms that are unchanged," the European Court of Justice said in a statement Monday. 

May's exit plan, which she agreed with EU officials following more than a year of torturous negotiations, aims to maintain broad trade ties on goods with the bloc, but allows for just enough freedom for the UK to strike new arrangements with countries such as the U.S. and China.

"I don't say that this deal is perfect. It was never going to be. That's the nature of a negotiation," May told lawmakers Tuesday. "We should not let the search for the perfect Brexit prevent a good Brexit that delivers for the British people."

Others have said that, owing to the mechanics of relationship between EU member Ireland and the U.K. British territory of Northern Ireland, her deal binds the U.K. into an arrangement that means it will have to follow EU rules without having a say in how they're made.

Opposition leader Jeremy Corbyn, who is attempting to use the Brexit chaos to force an early election, went a step further, calling May's deal a "Blind Brexit", arguing it gives too much power to the government to negotiate a future trade deal based on a vague framework of objectives agree with Brussels.

"We still don't know what our long term relationship with Europe would look like and that's way so many (lawmakers) are not willing to vote for this blindfold brexit and take a leap in the dark about Britain's future," he countered.

That view was given support last week by the publication -- against the Prime Minister's wishes -- of the full legal advice given to the government by Attorney General Geoffrey Cox when it reached its exit deal last month.

Cox warned that Britain could be locked into an EU customs union, preventing it from cutting trade deals elsewhere, and risks becoming subject to "protracted and repeating rounds" of negotiations with Brussels going forward.

Nigel Dodds, a member of the Democratic Unionist Party that props up May's government in a so-called "confidence arrangement", called the advice "devastating", cementing his vow to vote against the deal next week.