U.K. Prime Minister Theresa May was starting a frantic two-week race yesterday to convince the British public, and a skeptical Parliament, to back the Brexit deal she has struck with the European Union.

May was gathering her Cabinet for a meeting hours after returning from Brussels with the divorce agreement approved by the 27 other EU leaders.

The deal needs to be approved by Parliament, but scores of legislators — from both the opposition and May's governing Conservative Party — say they will oppose it. May aims to convince them that the deal "honors the referendum" in 2016 that saw Britain vote to leave the EU. She argues that the British people are sick of endless debates about Brexit, and backing the deal will allow "us to come together again as a country whichever way we voted." Parliament's vote is due before Christmas, likely the week of Dec. 10.

Brexit Secretary Stephen Barclay conceded that "it's going to be a challenging vote." But he said Britain would be in "choppy waters" if the deal was rejected. Rejection by Parliament would plunge Britain into a political crisis just weeks before it is due to leave the EU on March 29.

Both Britain and the EU are adamant that the U.K. can't renegotiate the deal, struck after 18 months of tense negotiations. May said "it is the best possible deal. It is the only deal." That hasn't stopped pro-Brexit campaigners pushing for a cleaner break from the bloc, and pro-EU activists trying to stop Britain leaving at all.

Yesterday, a European court threw out a challenge from 13 Britons living in other EU nations seeking the annulment of the Brexit negotiations. The complainants had asked the EU's General Court to annul the 2017 decision by member states to enter into Brexit talks, arguing that it was an implicit acceptance of Britain's intention to leave the EU. This would directly impact their EU rights, despite the fact that they had not been able to participate in the Brexit vote, they maintained.

Britain's 2016 referendum on EU membership was not open to British citizens who had been living in another member state for long enough to lose their voting rights at home.

The Luxembourg-based judges rejected their arguments, noting that the decision to open Brexit negotiations per se did not affect the complainants' legal situation and is "merely a preparatory act" that "draws the consequences" of Brexit. For this reason, the case was not admissible, the court added in a statement. In order for the court to rule on the case, the act being challenged "must, at the very least, be of direct concern to the applicants and directly affect their legal situation," the judges noted.

Only at the moment of Britain's departure from the EU would the rights of Britons living in the bloc be affected, the court found. However, this would be the case irrespective of whether a Brexit deal is struck, and not as a consequence of the decision to enter into talks. The decision can be appealed before the European Court of Justice, the EU's top tribunal.