Channel 4 has appointed the controller of digital network BBC Three as its new head of programming.
Bellamy started out as a producer on Dispatches and World in Action
Julian Bellamy returns to the commercial broadcaster, having spent seven years there from 1998 to 2005.
He will now oversee the commissioning process at Channel 4, deciding which shows are developed for broadcast.
There is also a promotion for Channel 4's director of television, Kevin Lygo, who gains greater power over sister channels E4, More4 and Film4.
He will also spend more time considering which material should appear on new-media services, such as online, mobile devices and DAB digital radio.
Bellamy, a former head of E4, said Channel 4 was the network that he had "grown up with personally and professionally".
But he added it was "impossible" to leave BBC Three "without a degree of regret".
Lygo has overseen TV shows Big Brother and Jamie's School Dinners
Jana Bennett, the director of BBC Vision, the department which includes television output, described Bellamy as "a brilliant member" of staff.
"In his time as controller of BBC Three, he's strengthened the channel, successfully growing its reach and approval among young adults. We all wish him well."
The BBC said Emma Swain, deputy controller of knowledge within BBC Vision, would be the acting controller of BBC Three until a permanent appointment was made.