Kennedy was born in 1919 in Edinburgh, the son of a career Royal Navy officer, Edward 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 had two younger sisters, Morar and Katherine. Morar married the playwright Royce Ryton in 1954. Katherine married Major Ion Calvocoressi in 1947. He was schooled at Eton College (where he played in a jazz band with Humphrey Lyttelton) and studied for a year at Christ Church, Oxford, until the outbreak of war. While at Oxford he was a member of the notorious Bullingdon Club.

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 Rawalpindi encountered two of the most powerful German warships, the small battleships (or battlecruisers) Scharnhorst and Gneisenau trying to break out through the GIUK gap into the Atlantic. Rawalpindi was able to signal the German ships’ location back to base. Despite being hopelessly outgunned, Kennedy decided to fight, rather than surrender as demanded by the Germans. Scharnhorst sank Rawalpindi; of her 312 crew, 275 (including her captain) were killed. Kennedy was posthumously mentioned in dispatches and his decision to fight against overwhelming odds entered the folklore of the Royal Navy. His son Ludovic was 20 years old.

Having studied for one year at Christ Church, Oxford, before the war, he returned to complete his studies in 1945. At Oxford he helped found the Writers’ Club and then sought a means of support while he completed a book on Nelson’s captains. After leaving Oxford he began a career as an investigative journalist.

In February 1950 he married the dancer and actress Moira Shearer in the Chapel Royal, Hampton Court Palace. He later remembered their meeting in 1949, when he was reluctantly persuaded by a friend to accept a complimentary ticket to a fancy dress ball held at the Lyceum ballroom in London. Shearer – who had recently become famous for her role in The Red Shoes – was presenting the prizes at the occasion, and Kennedy later recalled that “I felt a tremor run through me when I caught sight of her. She looked even lovelier than in the film.”

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 an increase in the Liberal vote, pushing the Conservatives into third place. The Rochdale contest was the first British by-election to receive live television coverage (locally, by Granada Television).

He wrote several books that questioned 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 1961 book Ten Rillington Place ( ISBN 978-0-586-03428-6). Evans was found to have murdered his baby daughter in 1950, but Kennedy contended that he was innocent, and that the murders of his wife and baby had been committed by the serial killer John Christie. Christie was hanged three years after the hanging of Evans, following the discovery of six more bodies at 10 Rillington Place, none of which could be ascribed to Evans. Indeed, two of the skeletons found at the house dated back to the war – long before Evans and his family had moved in. After a long campaign, Evans was posthumously pardoned in 1966. The scandal helped in the abolition of the death penalty in the UK. Kennedy’s book was filmed in 1970 as 10 Rillington Place, starring John Hurt as Evans and Richard Attenborough as Christie.

Ludovic Kennedy followed his father into the Royal 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. He witnessed the final battle, until Bismarck was ablaze and its crew began to abandon ship but shortage of fuel forced Tartar to depart for home before Bismarck sank. Kennedy later wrote about this in his 1974 book Pursuit, his chronicle of the chase and sinking of Bismarck.

In 1980 he presented an episode of the BBC television series Great Railway Journeys of the World, in which he crossed the United States.

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 on the political comedy series Yes Minister. Kennedy was the subject of an episode of That Reminds Me (2002: season 4, episode 1). Kennedy also expressed to another journalist that there were too many Blacks on television.

In 1985, Kennedy published The Airman and the Carpenter ( ISBN 978-0-670-80606-5), in which he argued that Richard 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.

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

In 1990, Kennedy became the advisory committee chairman of Just Television, a television production company dedicated to exposing miscarriages of justice.

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

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.

A lifelong atheist, he published All in the Mind: A Farewell To God 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 Scotland.

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 was 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 per cent of the vote and subsequently rejoined the Liberal Democrats.

In 2003, he wrote 36 Murders and 2 Immoral Earnings ( ISBN 978-1-86197-457-0), 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, police misconduct or 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.

Summoning up his courage, he approached the 23-year-old dancer and asked her to dance. She would be delighted, she told him, only “I don’t dance very well.” She was not, Kennedy revealed, a competent ballroom dancer. The couple had one son and three daughters (Alastair, Ailsa, Rachel and Fiona) from a 56-year marriage that ended with her death on 31 January 2006 at the age of 80.

Kennedy died of pneumonia in a nursing home in Salisbury, Wiltshire, on 18 October 2009, aged 89.