Brexit Secretary Dominic Raab quit the U.K. Cabinet over Theresa May’s handling of Brexit, dealing a severe blow to the prime minister who must now consider if her position is tenable. The pound tumbled.

Shortly afterward, Work and Pensions Secretary Esther McVey also resigned.

“Today, I have resigned as Brexit Secretary,” Raab said on Twitter. “I cannot in good conscience support the terms proposed for our deal.”

Raab’s departure is the big one May desperately wanted to avoid, and May’s own future -- and that of her Brexit plan -- are now in jeopardy. By quitting over her compromise position with the European Union on the U.K.’s departure, Raab has lined himself up to stand in a future leadership contest as a Brexit purist -- a position popular with a large tranche of Conservative lawmakers and grassroots members.

Raab is the second Brexit secretary to quit since May unveiled her blueprint for keeping close ties to the EU. David Davis -- who has been a friend and mentor to Raab -- resigned in protest for essentially the same reasons in July.

The resignations of Raab and McVey come hours after May said she’d secured “collective” Cabinet approval for her draft Brexit agreement, albeit making clear she expects significant battles ahead. McVey was said to be in tears at the meeting on Wednesday.

So the challenge of getting the deal through Parliament just got harder; freed from the collective responsibility of Cabinet, they can both now join Conservative rebels in voting against May’s plans.