Ashley Highfield, the chief executive of Project Kangaroo, the BBC/ITV/Channel 4 broadband TV joint venture, is to leave the endeavour before it launches.
Project Kangaroo was due to launch at Christmas, but has been delayed until mid-2009 at the earliest, while the service is investigated by the Competition Commission.
Highfield, the former BBC director of future media and technology, credited in part with the success of the iPlayer, only joined Project Kangaroo 4 months ago.
Highfield has been poached by Microsoft, where he will be managing director and vice-president of consumer and online UK.
"I was presented with an amazing opportunity which I couldn't turn down, and I am looking forward to embarking on my new challenge, while wishing everyone at the project continued success", he said.