Treasury official Michael Ellam is to be head of communications in Downing Street when Gordon Brown takes over as prime minister on 27 June.
Mr Ellam has worked for Mr Brown since he became chancellor in 1997, and for his predecessor Ken Clarke before that.
During a decade at the Treasury Mr Ellam, 38, has held numerous posts, including head of communications and strategy for three years from 2000.
He will handle daily briefings for the press as the PM's offical spokesman.
He will replace Tom Kelly who has been Tony Blair's official spokesman.
In 2004, Mr Ellam was appointed as director of policy and planning at the Treasury, with responsibility for short to medium-term coordination of policy.
News of his appointment comes on the same day as the Conservative party announced that former News of the World editor Andy Coulson is to become its director of communications.