BBC One controller Lorraine Heggessey is quitting the corporation to work for an independent television company.
Lorraine Heggessey was promoted to controller in 2000
Ms Heggessey, who has spent five years at the channel's helm, is to become chief executive at Talkback Thames, which makes The Bill and Pop Idol.
She is not expected to leave until later in the year.
BBC chairman Michael Grade paid tribute to the controller, saying: "Lorraine Heggessey will be a very tough act to follow."
Director general Mark Thompson, who appointed her, said on Monday he knew she had a "knack of recognising an original idea and making it work on screen".
The BBC's director of TV, Jana Bennett, said: "Lorraine has been a human dynamo in her successful effort to make BBC One the nation's favourite channel."
Six months ago, Ms Heggessey said she would like to be the longest-serving BBC One controller, remaining in the job for another two years at least.
Strictly Come Dancing has proved a ratings hit
But on Monday she said: "I feel I am ready for a new challenge and could not pass up the opportunity to join one of the UK's most respected production companies."
BBC people who are already being tipped for the controller's job include head of drama commissioning Jane Tranter, head of entertainment commissioning Jane Lush, head of entertainment and features Wayne Garvie, and head of daytime Alison Sharman.
Channel 4 director of programmes Kevin Lygo has also been linked with the job.
Ms Heggessey took over as controller in 2000 from Peter Salmon, and was promoted from head of the children's department and factual and learning.
She was the first woman to be appointed to the position.
Among the programmes commissioned under her leadership were the Blue Planet, Spooks and Strictly Come Dancing.
As well as successes, Ms Heggessey has had to deflect criticisms following a report in 2004 which remarked that an increasing number of people perceived the channel was declining in quality.
But she defended the channel saying it provided greater variety and breath of programming than any other channel in the UK, "possibly the world".
Halting the slide
The fall of EastEnders as the country's top-rated soap has also attracted harsh criticisms during her tenure, with plotlines and casting attracting scathing reviews in recent times.
An attempt to halt the slide by installing Kathleen Hutchinson as executive producer was deemed to have failed when Ms Hutchinson quit after just four months.
Talkback Thames is one of the country's leading independent production companies.
Talkback was set up by comedians Mel Smith and Griff Rhys Jones in 1981, and sold to media group Pearson in 2000 for £62m. It merged with Thames TV in 2003.
The current chief executive Peter Fincham is leaving the company in June, having been with the company for 20 years.