British Labour party leader Jeremy Corbyn, in Prague | Filip Singer/EPA Jeremy Corbyn loses key aide … to David Davis Labour leader’s economic adviser heads back to the civil service.

LONDON – Jeremy Corbyn’s head of economic policy quit to take a "middle ranking" job in David Davis’ Department for Exiting the European Union, senior Whitehall and Labour Party sources confirmed Friday.

Mike Hatchett, a former Treasury civil servant, joined Corbyn’s team barely a year ago and was highly regarded by Labour MPs and staff. One senior Labour official said he was “one of the grown ups” and his departure was a big blow to Corbyn.

Hatchett’s decision to "defect" back to government is likely to feed accusations that Corbyn is losing the support of all but his most loyal supporters. On Wednesday, Corbyn was one of the guests at the leaving party of his former campaign director Simon Fletcher, who quit last month after reportedly becoming disillusioned with the direction of the party.

Asked about Hatchett’s departure, a spokesman for the Labour leader declined to comment. However Corbyn’s allies, speaking on condition of anonymity, rejected the accusation that Hatchett had become disillusioned.

Hatchett, a specialist in financial services regulation and the European Union, had applied for a number of positions in government before being offered the position in the Brexit department, a senior Whitehall source said.

His experience in government, having served in the Treasury and the U.K.’s representation to the EU in Brussels, is seen as a perfect fit for the Brexit department, which has rapidly expanded since being created by Theresa May last year. Hatchett's appointment went through normal civil service channels and was not at a level that needed to be signed off by the Secretary of State.

A spokesman for the Brexit department said: "The department ensures all prospective staff fully understand the requirement to operate objectively and impartially under the Civil Service code.

"In addition to standard pre-employment checks, there are clear rules that individuals from a political background are subject to further checks on propriety and conflicts of interest.

"We are equipping the department with staff who have the right expertise to get the very best deal for the U.K."