Channel 5 director of programmes Dawn Airey is taking over from David Elstein as the broadcaster's chief executive, in a surprise move announced on Friday.
Mr Elstein, who became chief executive seven months before Channel 5 went on air in March 1997, will leave immediately and has no new job to go to at present.
Simultaneously, Channel 5 director of sales Nick Milligan has been promoted to deputy chief executive.
A key factor in the timing of the decision to replace Elstein is suggested to have been fears by Channel 5 shareholders that they might lose Airey who is known to have been approached recently by Granada Media.
Elstein was understood not to have seen eye to eye with Richard Eyre, director of strategy and content at the RTL Group, Channel 5's majority shareholder.
Elstein said his departure coincided with the completion of a four-year contract and would allow the channel to take the next step in its development.