Mr Karmazin was "frustrated" by Viacom's share performance

Mr Karmazin will now be replaced by both MTV Networks chief Tom Freston and CBS head Leslie Moonves.

Mr Redstone also announced he would be stepping down as head of the US group in three years' time.

However, he denied that Mr Karmazin had quit over their reported feud, saying his decision was spurred by frustration over the group's stock price.

Mr Redstone told reporters: "Nobody asked for Mel's resignation.

"Two weeks ago he told one of our executives that he was frustrated by the stock. That was the first time I ever heard him talk about resigning."

Chilly relations

In the past, reports have claimed that Mr Karmazin clashed with the Viacom head over various issues, such as the group's acquisition strategy, advertising sales strategy, the sluggish performance of its radio business and Mr Karmazin's sale of stock.

In a statement Mr Karmazin said he had left the company due to "personal and professional reasons", adding that he would now "pursue other interests".

There is no non-compete clause in the 60-year-old's contract, which is likely to spark speculation that he could join a rival media company such as Walt Disney.

The former president and chief operating officer suffered a number of problems at Viacom over the past 12 months when the group's radio division - his main area of responsibility - reported below-par results.

Mr Karmazin made his first foray into the industry through radio as a salesman before rising through the ranks.

James McGlyn , manager of the Summer Everest Fund in Cincinnati told Reuters: "It sounds like performance was the reason that Mel left, he wasn't bringing Sumner happy news."

However, while his departure marks the end of one chapter of chilly relations at the company, experts predict another will follow.

'Phenomenal growth'

The new co-presidents and co-chief operating officers - Mr Moonves and Mr Freston - could soon be competing to replace Mr Redstone in 2007.

Their new posts are viewed as a reward for the strong performances of their respective divisions.

Mr Moonves is credited with bringing strong TV brands such as Survivor and crime show CSI to the market, while Mr Freston's MTV has been a lucrative money draw around the world.

Mr Redstone said the pair had both "presided over phenomenal growth stories ... and each has served as an integral member of Viacom's senior management team".

He also hinted that the one of the two will be his likely successor, adding he would be "amazed" if an outsider took control of the group.

Analysts were unsurprised by the move.

"It was important that they showed their bench strength by naming both Freston and Moonves," Angela Kohler, of Pittsburgh's Federated Investors said.