However, the prime minister has stressed that does not mean a reversal of Brexit.

By accepting the role, Mr Ellam effectively takes over from Sir Olly Robbins, who led Brexit negotiations for Theresa May and whose return to the civil service as the head of the Foreign Office was announced last week.

The advert for the job said that the position would "receive significant public scrutiny and public attention".

Mr Ellam worked as a civil servant for 20 years including a two year stint as Downing Street's director of communications, when Gordon Brown was prime minister.

Cabinet Office minister Nick Thomas-Symonds said: "Michael's experience at the highest levels of international finance will be a huge asset as we work to deliver economic growth on behalf of the British people, including through resetting our relationship with the EU."

Former Chancellor George Osborne, who worked with Mr Ellam at the Treasury, described him as "one of the sharpest, most strategic minds, in the best traditions of the impartial civil service".