This afternoon’s news of Jo Johnson’s resignation as Rail Minister throws into sharp relief the challenges still facing Theresa May in getting any kind of Brexit deal through Parliament. Confidence has grown over the past fortnight that a deal is likely to be struck between the UK government and the European Commission / remaining EU Member States. However, the parliamentary arithmetic that the government will need to confront back in Westminster if and when they secure an agreement remains daunting.

Johnson’s resignation follows hot on the heels of reports in The Times that May’s attempts to reassure the DUP have simply increased their suspicions that she is about to sign up to a deal that retains some Irish border backstop element that will be unacceptable to them. If the Prime Minister is unable to rely on the support of the DUP, on top of the anticipated opposition of hardcore Brexiteers, the numbers start to look very difficult indeed.

It remains to be seen whether Johnson’s resignation will be an isolated incident, or whether further resignations will follow in the coming days. If it is the former then No 10 will hope to ride this out, as they did following the other Johnson resignation earlier in the year. If it is the latter, then this could be the start of a very uncomfortable week indeed for the government.

Either way, there is no escaping the significance of the fact that Johnson was a remain supporter and his resignation is a clear indication that May’s Brexit strategy is pleasing very few people indeed. Of course, there is still a feeling in No 10 and elsewhere that the spectre of a chaotic no-deal will ultimately prove too scary an alternative for the majority of MPs, but this approach represents a high degree of gamesmanship that will leave many in the business community feeling very nervous.

What is clear, is that as the negotiations enter the final straight (at least over the withdrawal agreement) there is an enormous job facing the Prime Minister to sell whatever emerges from the negotiation to Parliament. That job is getting harder by the day.