David Liddiment, programming boss at the UK's leading commercial channel ITV1, is to resign after five years in the job.
He will leave towards the end of the year once a successor has been appointed.
News of his planned departure has shocked colleagues and competitors alike as Liddiment is highly regarded in the industry. Under Liddiment, ITV1 has been a formidable competitor in the ratings battle with the BBC - he has commissioned a string of hit shows which have buoyed the network including Who Wants To Be A Millionaire', Pop Idol and Cold Feet.
However, things have become increasingly difficult this year. The advertising downturn has hit ITV hard, hitting programming budgets. And the closure ITV Digital has distracted the attention of ITV shareholders and financiers Granada and Carlton.
ITV1 has also been defeated in the ratings this year for the first time by BBC1, partly a result of the shock defection of daytime stars Richard Madeley and Judy Finnigan and the disastrous scheduling of football show The Premiership at 7pm.
Liddiment commented: "Despite the financial and competitive challenges we have faced most recently, there is a terrific spirit alive in ITV that I know will see it thrive in the future."
Clive Jones, ITV joint managing director commented: "David is without doubt one of the finest programme directors of his generation. It's a mark of his commitment to ITV that he is managing his departure so carefully and we thank him for that."