Boris Johnson resigned from the U.K. government, tipping Prime Minister Theresa May deeper into crisis and increasing the chances she’ll face a leadership challenge over her Brexit policy.

Johnson, who served as foreign secretary, was the face of the Brexit campaign. His exit sends a strong message to the 52 per cent of voters who backed leaving the European Union in 2016 that their decision is being betrayed. It’s also just hours since Brexit Secretary David Davis quit in protest over May’s efforts to keep Britain closely bound to the EU after the split, due in March.

High-profile resignations can unleash leadership challenges in the U.K., and Davis’s prompted pro-Brexit lawmakers to demand May rip up her proposals -- though they didn’t call for her to go. That’s partly because they can’t be sure they have the numbers needed to defeat her.

But Johnson’s exit could embolden others.

Hardline Brexit backers in the Conservative Party are meeting later Monday to decide on their strategy. May’s decision to brief Labour lawmakers on her Brexit plan, an apparent effort to try to count on opposition votes if her own side lets her down, has infuriated euroskeptic Tories.

Just on Friday, May announced she had secured the backing of her whole Cabinet for her Brexit plan and told ministers to either back it or resign. It was a rare consensus, following talks at her Chequers country retreat about the way forward -- a move meant to kick-start talks with the EU that have been stalled for months.

Ministers signed off on a blueprint for a new U.K.-EU “free trade area,” with interwoven customs regimes, and identical regulations for industrial and agri-food goods. Critics in May’s Conservative Party said the plan would bind Britain to EU rules and prevent signing trade deals with countries outside the bloc.

Johnson was said to have decided not to quit over May’s soft Brexit plan because he wanted to remain in Cabinet and fight for the divorce he wants, a person familiar with his thinking said after the Chequers meeting.

But criticism from euroskeptic Tories mounted over the weekend, with many calling for ministers to stand by their principles and make a stand.

First Davis and then Johnson appear to have done so, and their resignations have come at a critical and highly sensitive time for May. A divorce agreement with the EU is due to be wrapped up in October, but there are still major obstacles to overcome.