Beth Russell, one of Brown's most trusted aides, will return to the Treasury in late July.
She is the third member of Brown's inner circle to announce their departure this year.
Number 10 permanent secretary Jeremy Heywood is now trawling government departments to find a replacement for Russell, who has been a member of "team Brown" for nearly a decade.
PRWeek has seen a letter that Heywood recently sent to all permanent secretaries asking them to put forward suitable candidates.
In the letter, dated 17 June, Heywood describes the job as ‘a high profile and demanding post involving daily contact with the Prime Minister'.
The letter goes on: ‘I would be grateful if you could each put forward the names of one or two individuals who would particularly excel in this role.'
Heywood's trawl suggests that Downing Street is struggling to fill the role.
One Number 10 insider said: ‘Normally people would just walk into a role like this. The fact that Heywood is issuing this letter to all permanent secretaries suggests desperation.'
Another senior source said: ‘Beth has done the best possible job in very hard circumstances. The trouble with Gordon's speeches isn't the speechwriter - the clunkiest lines are the ones he writes himself.'
PRWeek recently revealed that Downing Street had approached a trio of Blairite speechwriters amid concern that Brown's big speeches were failing to resonate with the public and told how Brown has increasingly been relying on his close friend Colin Currie - a doctor by trade - to draft his speeches.
Changes at No 10
July 08 Chief speechwriter Beth Russell to depart
June 08 Political secretary Fiona Gordon departs
April 08 ‘Visual gaffe monitor' Nicola Burdett enters. Head of digital comms Mark Flanagan enters
March 08 Strategy director Spencer Livermore departs.Strategy director David Muir and strategic comms adviser Nick Stace enter
Jan 08 Chief of strategy Stephen Carter enters