Ludovic Kennedy, the man who helped end capital punishment

Ludovic Kennedy, one of the leading broadcasters and writers of his generation, has died of pneumonia in a Wiltshire nursing home, aged 89.

Sir Ludovic probably reached his biggest audience in November 1982, when he appeared in the popular sitcom Yes Minister . In the episode entitled "The Challenge", the hapless minister Jim Hacker agrees to be interviewed by "my friend Ludo", thinking that he will be given an easy time, only to be subjected to a John Humphrys-style grilling with such potentially disastrous results that Hacker and his civil servants spend the rest of the episode making sure the interview is never broadcast.

It was a funny interlude in the life of an intensely serious man who, outside his professional life, spent a great deal of time campaigning for the causes in which he believed.

They included the right to die at the time of your choosing. A humanist, he was president of the Voluntary Euthanasia Society and resigned from the Liberal Democrat party in 2001 when its then leader, Charles Kennedy, refused to endorse assisted dying. He had supported the party and its forerunner, the Liberal party, for decades, standing as a Liberal candidate in 1958, but fought the 2001 election as an independent in Devizes, Wiltshire, and collected a respectable 1,078 votes. He later rejoined the party.

"Ludovic Kennedy was one of the great thinkers of his generation," the Liberal Democrat leader, Nick Clegg, said yesterday. "His pursuit of justice and his championing of sometimes unpopular and controversial causes marked him out as a true liberal. He will be greatly missed."

As a writer Kennedy was famous – and to some notorious – for revisiting high-profile criminal cases where he suspected a miscarriage of justice. His most successful book was 10 Rillington Place, in which he argued that Timothy Evans, hanged in 1950 for the murder of his infant daughter, was innocent and that the murder was actually carried out by the serial killer John Christie, who lived at the same address. The book not only achieved a posthumous acquittal for Evans, it also helped bring about the abolition of capital punishment in Britain in 1965.

By then Kennedy was a well-known face in Britain, as a presenter of BBC's Panorama, and later as an ITN newsreader. He was knighted in 1994 for services to journalism.

He also campaigned for a posthumous pardon for Derek Bentley, who was hanged in 1953 for the murder of a policeman even though he was being restrained by police when an accomplice shot dead PC Sidney Miles.

And he challenged the reputation of the FBI by revisiting one of the most celebrated cases in US criminal history to argue that Bruno Richard Hauptmann, executed in 1936 for the kidnap and murder of the baby son of the aviator Charles Lindbergh, was also innocent.

One of his last books, published in 1999, was All in the Mind: Farewell to God, in which he argued that God was a creation of the human imagination, rather than the opposite.

Ludovic Kennedy was born in Edinburgh in 1919. As a schoolboy at Eton, he played in a jazz band with Humphrey Lyttleton. He joined the navy as a young man, and his ship HMS Tartar was involved in the sinking of the German battleship Bismarck. He was married for 56 years to the ballet dancer Moira Shearer, who died in 2006. They had a son and three daughters.

Hanne Stinson, chief executive of the British Humanist Society, said yesterday: "Sir Ludovic was a stalwart supporter of the BHA and a progressive campaigner on many fronts. He will be sorely missed."