Kennedy was born in Edinburgh, Scotland, the son of a career Royal Navy officer, Edward Coverley Kennedy, and his wife, Rosalind Grant, daughter of Sir Ludovic Grant, 11th Baronet. His mother Rosalind was a cousin of the Conservative politician Robert Boothby, later Lord Boothby. He was schooled at Eton College (where he played in a jazz band with Humphrey Lyttelton), and was set for university when the Second World War broke out.

Kennedy's father, by then a 60 year old retired captain, returned to the navy and was given command of HMS Rawalpindi, a hastily militarised P&O steamship, known as an Armed Merchant Cruiser. On 23 November 1939, while on patrol southeast of Iceland the Rawalpindi encountered two of the most powerful German warships, the battlecruisers and trying to break out through the GIUK gap into the Atlantic. The Rawalpindi was able to signal the German ships' location back to base. Despite being hopelessly outgunned, Captain Edward Coverley Kennedy of the Rawalpindi decided to fight, rather than surrender as demanded by the Germans. Scharnhorst sank Rawalpindi; of her 312 crew 275 (including her captain) were killed. His son Ludovic was just twenty years old. Captain Kennedy was posthumously Mentioned in Despatches and his decision to fight against overwhelming odds entered the folklore of the Royal Navy.

Ludovic Kennedy followed his father into the navy; he served as an officer on destroyers, mostly in the same northern seas. His ship (HMS Tartar) was one of those that pursued the battleship Bismarck following the Battle of the Denmark Strait although he did not witness her sinking because Tartar went to refuel some hours before the end. Kennedy later wrote about this in Pursuit, his chronicle of the chase and sinking of the Bismarck.

He had three younger sisters, Morar, Edna and Katherine. Morar married the playwright Royce Ryton in 1954. Katherine married Major Ion Calvocoressi in 1947.

After the war he attended Christ Church, Oxford, and began a career as a journalist.

A campaigning, investigative reporter, Kennedy wrote for a number of publications, including Newsweek. From 1953, he edited and introduced the First Reading radio series on the BBC Third Programme, presenting young writers such as Kingsley Amis and Philip Larkin. Later he became a television journalist and a newsreader on ITV's Independent Television News. He presented the BBC's flagship current affairs programme Panorama for several years. Kennedy was interested in miscarriages of justice, and he wrote and broadcast on numerous cases.

From 1980 to 1988 he presented the television review programme Did You See...? He interviewed Peter Cook's character Sir Arthur Streeb-Greebling in A Life in Pieces in 1990. He appeared as himself in several episodes of the political comedy series Yes Minister, and was the subject of an episode of That Reminds Me. Private Eye magazine sometimes referred to him as 'Ludicrous Kennedy'. In the long-running BBC sitcom Till Death Us Do Part, Alf Garnett - while attacking BBC personalities - spoke of him as a Russian Mick ("Mick" being an offensive term for an Irishman), meaning "that Ludovich Kennedy!"

Kennedy's highly regarded book Pursuit: The Chase and Sinking of the "Bismarck" (ISBN 0-304-35526-7) detailed the career of the Bismarck, her sinking of British battlecruiser Hood, and her destruction by the Royal Navy.

Miscarriages of justice

He wrote several books that question convictions in a number of notable cases in British criminal history. One of the first miscarriages of justice he investigated was the conviction and hanging of Timothy Evans in his book Ten Rillington Place (ISBN 0-586-03428-5). He was found to have murdered his baby daughter but Kennedy contended that Evans was innocent, and that the crimes had been committed by the serial killer John Christie. Christie was hanged several years after the hanging of Evans, following the discovery of several more bodies at 10 Rillington Place, none of which be ascribed to Evans, and as a result of a long campaign, Evans was posthumously pardoned. The scandal helped in the abolition of the death penalty in the UK. Kennedy's book was filmed in 1970: Richard Fleischer's film 10 Rillington Place starred John Hurt as Evans and Richard Attenborough as Christie.

In 1985, Kennedy published The Airman And The Carpenter (ISBN 0-670-80606-4), in which he argued that Bruno Hauptmann did not kidnap and murder Charles Lindbergh's baby, a crime for which he was executed in 1936. The book was made into a 1996 HBO film Crime Of The Century, starring Stephen Rea and Isabella Rossellini.

In 2003, he wrote 36 Murders and 2 Immoral Earnings (ISBN 1-86197-457-4), in which he analysed a number of noted cases, including the Evans case and those of Derek Bentley and the Birmingham Six, a number of which were affected by claims of police failure or downright perjury. In it he concluded that the adversarial system of justice in the UK and the United States "is an invitation to the police to commit perjury, which they frequently do", and said that he preferred the inquisitorial system.

Kennedy also wrote:

• Sub-Lieutenant: A Personal Record of the War at Sea, 1942
• One man's meat, 1953
• Murder Story, 1954
• Trial of Stephen Ward, 1964, ISBN 0-575-01035-5
• Very lovely people; a personal look at some Americans living abroad, 1969, ISBN 0-671-20205-7
• Nelson and His Captains (also called Nelson's band of brothers), 1975, ISBN 0-00-211569-7
• Presumption of Innocence: Amazing Case of Patrick Meehan, 1976, ISBN 0-575-02072-5
• Death of the Tirpitz (also called Menace ... The Life and Death of the Tirpitz), 1979, ISBN 0-316-48905-0
• On My Way to the Club, 1990, ISBN 0-00-637079-9 (his autobiography)
• Truth to Tell: Collected Writings of Ludovic Kennedy, 1992, ISBN 0-552-99505-3
• In Bed with an Elephant: Personal View of Scotland, 1995, ISBN 0-593-02326-9
• All in the Mind: A Farewell To God, 1999, ISBN 0-340-68063-6 (a critique of Christianity)

In 1958, Kennedy stood for election to Parliament as the Liberal candidate in the Rochdale by-election called after the death of the sitting Conservative MP, Wentworth Schofield in December 1957. He lost to the Labour candidate, Jack McCann, but achieved a massive increase in the Liberal vote, pushing the Conservatives into a distant third place. The Rochdale contest was the first British by-election to receive live television coverage (locally, by Granada Television).

Kennedy supported Scottish Independence.

In addition to his writing and campaigning on miscarriages of justice, Kennedy campaigned on a number of other issues.

A lifelong atheist, he published All In The Mind: A Farewell To God (ISBN 0-340-68063-6) in 1999, in which he discussed his philosophical objections to religion, and the ills he felt had come from Christianity. He was a Distinguished Supporter of the British Humanist Association, he contributed to New Humanist magazine, he was an Honorary Associate of the National Secular Society and a Distinguished Supporter of the Humanist Society of Scotland.

He was also an advocate of the legalisation of assisted suicide, and is a co-founder and former chair of the Voluntary Euthanasia Society. His book, Euthanasia: The Case for the Good Death (ISBN 0-7011-3639-1), was published in 1990.

Kennedy resigned from the Liberal Democrats in 2001, citing the incompatibility of his pro-voluntary euthanasia views with those of the then Liberal Democrat leader Charles Kennedy (no relation) who is a Roman Catholic.

He then stood as an independent on a platform of legalising voluntary euthanasia in the 2001 general election for the Wiltshire constituency of Devizes. He won 2% of the vote and subsequently rejoined the Liberal Democrats.

In February 1950 he married the dancer and actress Moira Shearer in the Chapel Royal, Hampton Court Palace. The couple went on to have one son and three daughters from a 56-year marriage that ended with her death on 31 January 2006 at the age of 80.

He received an honorary doctorate from the University of Strathclyde in 1985.

He was knighted in 1994 for services to journalism, on the recommendation of John Major's government. Major's predecessor Margaret Thatcher had vetoed Kennedy's knighthood.

Kennedy died of pneumonia in a nursing home in Salisbury, Wiltshire, on 18 October 2009.