Channel 4's director of television Tim Gardam has announced his resignation after five years in the post.
Mr Gardam is leaving at the end of the year
Mr Gardam told staff at the London channel's headquarters on Wednesday he would leave his job at the end of 2003.
Mr Gardam said he was stepping down for "personal reasons" and because he wanted to do something new.
"I am announcing it now because I want to ensure a smooth succession," Mr Gardam said.
"It leaves me free to consider various options without it causing uncertainty here.
"I will have been director of programmes and television for five years this autumn, and this has always been the maximum time I thought it sensible to do the job. I now want to do something new."
In his time at Channel 4, Mr Gardam - who is also director of programmes - has overseen the introduction of some of its biggest hits, including Big Brother, Faking It and Peter Kay's Phoenix Nights.
His replacement will be appointed later this year and Kevin Lygo, director of programmes at Five (Channel 5), is considered a strong candidate.
Big Brother was brought to the channel by Mr Gardam
He worked for Channel 4 for three years as head of arts and entertainment before moving to Five.
Mr Gardam's career has seen him move in the other direction, as he was Five's director of programmes before going to Channel 4.
He was recruited by Michael Jackson, the channel's then chief executive. Mr Jackson was succeeded by Mark Thompson.
The loss of Mr Gardam - a former head of the BBC's weekly news programmes - will be a seen as a blow for the channel by
many in the television industry.
Under his leadership, many of the channel's entertainment programmes have become some of the UK's most popular programmes.
Programmes such as satirical news show Brass Eye are also considered by many critics as some of the most innovative in recent years.
Mr Gardam also turned Channel 4 News into a seven-day bulletin and outbid the BBC and Five for The Simpsons in 2002.
However, Mr Gardam and Mr Thompson have also been accused of lowering standards at the channel with the introduction of several so-called "reality TV" shows.
Earlier this month, Mr Gardam called for an end to the BBC's monopoly over the licence-fee income.
He called for National Lottery-style funding to help terrestrial broadcasters compete with the corporation.