One of Brexit Secretary David Davis’s key officials has quit his job to go and work for Jeremy Corbyn instead.

Mike Hatchett, a civil service expert in EU financial regulation, has been appointed as Labour’s Director of Policy and Research, HuffPost has been told.

His appointment is viewed by party insiders as “a real coup”, as well as an indictment on the current state of the Government’s Brexit plans.

One Whitehall source said that it was highly unusual for a Government official in such a sensitive department to defect directly to the Opposition.

Hatchett, a former Treasury official who only joined Davis’ department last October, has had strict “conditions” applied to his move to Labour.

He is bound by the Official Secrets Act not to divulge any details of the Government’s plans, and his responsibilities under the Civil Service Code and Whitehall rules on outside appointments.

The policy expert had previously worked for Corbyn as head of economic policy and his arrival at the Department for Exiting the EU (DExEU) in 2017 was seen as a blow to Labour. 

A spokesperson for DExEU said: “Mike Hatchett works for DExEU as a middle-ranking civil servant. In line with policy, he has informed us that he wishes to take up a new role outside of Government.

“Under the Government Business Appointment rules we have reached an agreement with Mr Hatchett and placed a series of conditions regarding this appointment, which he and his future employer are aware of, as per the usual protocols of this policy.”

One departmental insider played down the departure, stressing that Hatchett had no direct role in the negotiation team with Brussels.

But for Labour, Hatchett’s appointment is seen as a boost as he replaces long-standing party HQ official, Simon Jackson.

Hatchett was well-liked by MPs and staff when he worked for Corbyn directly during his previous stint with the party.

He has worked in the UK Permanent Representation to the EU, known as ‘UKRep’, in Brussels and is now seen as one of the party’s best assets on the detail of European policy. 

A Labour source said that Hatchett’s appointment had been confirmed in recent days. He will work closely Corbyn’s head of policy Andrew Fisher in a bid to better join up links between the party HQ and the leader’s office.

“Getting Mike back is a real coup for the party. He brings a calm head and a huge weight of knowledge,” one source told HuffPost.

Corbyn has caught Theresa May off guard in recent weeks at Prime Minister’s Questions, jibing her about Cabinet splits over Brexit as the the EU ridiculed the UK’s two main customs proposals.

Labour is itself trying to heal its own divisions, but MPs have noted a hardening of opposition to May’s Brexit plans and are hopeful that they will end up voting against the final deal.

The party has been expanding its staffing teams both at its HQ and in Corbyn’s office in recent months.

Carl Shoben, a former Blair-era media adviser, was appointed as the party’s new Director of Strategy, while former senior BBC News executive Ajula Singh was made Director of Communications.