LONDON -- Chris Patten, chairman of the BBC's governing body, the BBC Trust, has announced that he will be stepping down with immediate effect on health grounds following major heart surgery.

Patten's term as chairman was due to end in April 2015. He took up the post in May 2011.

Vice chairman Diane Coyle will take over as acting chairman until a successor is appointed.

In the past few years, the BBC has been dogged by scandal, including allegations of serial sex abuse by veteran presenter Jimmy Savile, IT bungles and overly generous executive payoffs.

Patten oversaw the appointment of George Entwistle as director-general, the BBC's editorial chief, in September 2012, but Entwistle resigned 54 days later following allegations that the BBC had tried to cover up evidence of Savile's activities and a false accusation of sex abuse made by a BBC program against a senior member of the ruling Conservative party.

Throughout this rocky period, Patten, himself a former politician for the Conservative party, has defended the public broadcaster to the hilt.

Patten said in a statement: "The BBC is a huge national asset, which is part of the everyday fabric of our lives. It is not perfect -- what institution is? It always needs to challenge itself to improve. But it is a precious and wonderful thing, a hugely positive influence, which benefits greatly from the creativity and dedication of its staff.

"I have had no reason to doubt that the leaders of all main political parties support the role it plays at the center of our public realm.

"Most important of all, the British public enormously value the strength of its output, its independence and the contribution it makes every day to the quality of our lives."

Tony Hall, the BBC's current director-general, said in a statement: "He is a staunch believer in the BBC, and he has brought his vast experience to the role of chairman of the BBC Trust. He has steered the BBC through some of its most difficult days. In undertaking this role he brought unrivalled experience, wisdom, and an overwhelming desire to ensure that the BBC remains the best public service broadcaster in the world."

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