The Prime Minister’s Director of Political Strategy is reportedly leaving to take a job in advertising.
Spencer Livermore was named the most important gay person in British politics in the PinkNews.co.uk Top 50 list in December.
His departure is seen as a blow to Gordon Brown.
Mr Livermore has been one of his closest and most loyal aides for more than a decade.
The Daily Telegraph reports that the Prime Minister and the Secretary of State for Children, Families and Schools, Ed Balls, had both spent the past few weeks trying to persuade Mr Livermore to stay.
The LSE graduate, who is still only 32, will take up a role working with ad company Saatchi Saatchi and Fallon working on the Labour party account.