After it looked as if Theresa May had finally united her Cabinet and could now govern with some authority, the resignation of David Davis MP puts her in, arguably, the most vulnerable position she’s been in since becoming the Conservative Party leader.

This resignation has cast doubt over the whole Brexit process, with some now calling for the Prime Minister to change her Brexit policy, whilst others are now calling for a second EU referendum. Mrs. May will be coming before the powerful 1922 Conservative backbench committee later in the day and it is here that she will have to convince her own backbenchers she still has what it takes to lead.

Brexit Secretary resignation: Why now?

After the 12-hour-long Cabinet meeting on Friday to agree on a Brexit policy, Mr. Davis eventually came to the conclusion that he could no longer continue in his role as he did not “believe” in the Prime Minister’s plan, as reported by the BBC. In his resignation letter, he went on to say that it was looking “less and less likely” that the Conservatives would deliver on the Brexit they promised to the people; that of leaving the customs union and the single market, according to Sky News.

He said that the current proposal by the PM would leave the UK weak in its negotiating position and said that he felt the UK had already made too many easy concessions to the EU.

He ended his letter by saying that Theresa May needed “an enthusiastic believer” in the approach, not merely a “reluctant conscript”, as reported by Sky News. It is now the task of the new Brexit Secretary, Dominic Raab MP, who was a prominent ‘Leaver’ in the referendum campaign, to move forward with the government’s Brexit plan and push their legislation through Parliament.

Another government minister at the Department for Exiting the EU, Steve Baker MP, has also resigned. He played a leading role in the 2016 referendum and was a close ally to David Davis. The real question now is whether any other Cabinet ministers will bite the bullet and resign too. The Cabinet’s agreed upon stance is much more favourable to pro-EU MPs than to Brexiteers, and thus some are looking to the likes of Boris Johnson and Michael Gove, to see whether they too will resign out of principle.

Theresa May vulnerable: The beginning of the end for the PM?

This resignation is a massive blow for Mrs. May. Just as it looked as if the Cabinet were putting their divisions to one side and finally acting like a collective body once again, the chief minister associated with Brexit creates yet more uncertainty and instability. Some are likening it to the resignation of Michael Heseltine in 1986 over the Westland Affair, which did eventually result in a leadership challenge.

Although David Davis has ruled out any such challenge from himself, it is certainly not out of the question that another Conservative MP may revolt against the PM. The European Research Group, led by Jacob Rees-Mogg MP, is a group of Brexiteer MPs in the Tory party, and it is thought that if a challenge were to come to Theresa May’s leadership, it would be from this group.

Thus, as Mrs. May is set to address Tory backbenchers in the evening to brief them on the Cabinet’s Brexit policy, it is crucial she wins their support; if not, she could be facing a challenge to her leadership sooner rather than later.