A ministerial aide to British Foreign Secretary Boris Johnson resigned Friday, in what appeared to be a stunning blow to Prime Minister Theresa May, as the government’s chief whip, Julian Smith, said the aide’s letter of resignation “singled out and directly questioned” the Brexit deadline.

Richard Harrington, a junior Cabinet Office minister, said he could not guarantee Britain would leave the European Union on time.

“Brexit is a hard and dangerous job, and I should not be expected to devote myself to it in the face of such constant and unpredictable challenges,” Harrington wrote.

He was the latest in a wave of resignations in the wake of May’s decision to force lawmakers into a vote that critics say the government has no chance of winning.

The vote, expected later this month, is meant to ask whether parliament wants a softer approach to Brexit, calling for a softer split from the European Union in a special agreement or saying that the exit should be “neither hard nor soft, but fair and orderly.”

There was speculation Thursday that an attack by Johnson over leadership speculation might influence Harrington’s decision to quit. “Boris was in my office at around 11:30 yesterday afternoon,” Labour lawmaker Pat McFadden told the Guardian. “He was very serious, with the idea of literally going on a ranting flight of stairs because these allegations were without foundation.”

Johnson, the leader of the Conservative Party, made the timing of the vote an election issue. “Tomorrow, parliament will have the chance to vote to say we should not crash out of the EU on 29 March without a deal, or alternatively take back control without a political agreement,” he said in a pre-recorded address to American CEOs at a Brexit conference in Berlin.

Harrington’s resignation, in particular, was seen as a possible signal to Johnson and others not to back down on the vote.

Johnson’s spokesman said that he has “no role” in Harrington’s resignation.