The Labour rulebook states that anyone who supports a rival party “shall automatically be ineligible to be or remain a party member,” and as a result Fisher has had his membership suspended pending an investigation by Labour HQ following complaints from Labour MPs and a former candidate.

But although it might appear that this is a battle over how the party’s arcane rulebook is interpreted in relation to one member of Corbyn’s staff, the case is being used as a test case by the leader’s internal Labour opponents to see how much power they can exert over the leader using existing party structures.

Essentially, they want to find out who is actually in control of the party: Corbyn and his hundreds of thousands of supporters among the ordinary Labour party membership, or the predominantly anti-Corbyn MPs and party staff.

As a result, when Labour HQ announced the suspension, Corbyn took the unusual move of issuing a statement alongside the announcement, giving his full backing to Fisher. He also said Fisher would continue to work in his office and set Labour policy, even while suspended from the party.

“I have full confidence in Andrew Fisher and his work,” said Corbyn, putting substantial pressure on Labour HQ and the party's national executive committee to clear his policy chief of any wrongdoing. “I respect the integrity of the general secretary’s office and trust that this matter will be settled as quickly as possible.”

In a tit-for-tat war fought largely through the medium of screengrabs of social media posts, Fisher's supporters have responded by highlighting a Facebook post by one of his accusers: former Labour candidate Emily Benn. In it she suggests people may wish to back the Women's Equality Party, which intends to stand candidates against Labour but has yet to do so.

