Sir Gus O'Donnell has been at the heart of government in Britain for 30 years. He was John Major's press secretary. Under Tony Blair he took on the top job in the civil service - a position he held when Gordon Brown took over.
At the last election he oversaw the negotiations for Britain's first full coalition government since World War II.
After nearly two years with David Cameron as prime minister he has now retired. Sarah Montague asks him if civil servants have too much power?
You can watch the full interview on BBC World News on Thursday 19th January at 0430, 0930, and 1530 and on BBC News Channel on Thursday 19th January at 0430 GMT, Friday 20th at 0030 GMT.
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