Why was Lord Mountbatten, Prince Charles’s great-uncle, assassinated?

At 11.30am, Mountbatten boarded Shadow V along with his daughter Patricia and her film-producer husband, John; John’s mother, Doreen Knatchbull, Lady Brabourne; and Patricia’s 14-year-old twins, Nicholas and Timothy, together with 15-year-old Paul Maxwell (who holidayed in the village and helped with the boat). With two Garda detectives following the progress of the boat through binoculars from the shore, the boat cleared the harbour wall and headed for the open bay. Mountbatten, standing tall at the wheel, opened the throttle to gain speed. Also watching the progress of the boat through binoculars were another two pairs of eyes – belonging to members of the Provisional IRA.

At exactly 11.45am, just as their boat reached the lobster pots, a few hundred yards away on the cliff top overlooking the bay, the PIRA team pressed the button which activated the bomb they had planted on the boat, moored in the bay, the night before. Fifty pounds of gelignite exploded, sending showers of timber, metal, cushions, lifejackets and shoes into the air. Then, there was a deadly silence.

Mountbatten’s body, his legs severed and most of his clothes ripped off in the blast, except for a fragment of his long-sleeved jersey with the badge of HMS Kelly on the front, was found floating face downwards in the water. He had been killed instantly together with Nicholas and Paul – Doreen later died of her wounds. Timothy survived the attack.

• Read more: Lord Mountbatten: Did Prince Philip’s uncle attempt to lead a coup against Harold Wilson’s government?

There was further carnage that afternoon when the IRA ambushed a British Army convoy and then also the troops sent to reinforce it just over 100 miles away on the Irish border at Warrenpoint. Eighteen British soldiers were killed and six were seriously injured, making it the deadliest attack on the British Army during The Troubles.

The Provisional IRA, admitting responsibility for the attack, released a statement: “This operation is one of the discriminate ways we can bring to the attention of the English people the continuing occupation of our country.”

• Netflix’s The Crown: the real history behind the royal drama

Prince Charles, who regarded Mountbatten as his ‘honorary grandfather’, was told the news while fishing in Iceland. That evening, he wrote in his journal “Life will never be the same now that he has gone.”

• Listen | Andrew Lownie discusses the colourful and controversial lives of Louis and Edwina Mountbatten

Mountbatten’s funeral

Mountbatten’s funeral, codenamed Operation Freeman, took place on 5 September at Westminster Abbey with 1,400 guests including Queen Elizabeth II, and a 30-minute address given by Prince Charles, which was televised in more than 20 countries.

On 5 November the trial of Thomas McMahon, aged 31, one of the Provisional IRA’s most experienced bombmakers, and Francis McGirl, aged 24, a known PIRA activist, opened. The two men, who had been acting suspiciously, had been taken in for questioning at a routine road check even before the explosion. The evidence against them was circumstantial – the men had traces of gelignite on them and McMahon had specks of green paint from Shadow V.

McMahon served 19 years before being released under the Good Friday Agreement, while McGirl was released as there was insufficient evidence to convict. He was crushed to death by a tractor in a mysterious accident in 1995. It is believed the assassination team actually comprised six people, including several women as watchers, but there has never been sufficient evidence to charge them.

• Read more: Did Mountbatten really leave 500 pages of instructions for his funeral?

Why was Lord Mountbatten murdered?

Mountbatten had been an IRA target since the early 1960s and there had been several attempts to kill him in the past. In 1978 an attempt to shoot him on Shadow V had been aborted when choppy seas prevented the sniper lining up his target, and the same year a loosened bung was found on the boat.

The Shadow Secretary of State for Northern Ireland, Airey Neave, had been assassinated by the Irish National Liberation Army in March 1979 and Mountbatten had been warned by those responsible for his protection that there was a plot to assassinate a member of the royal family – probably him – and he was told not to go to Ireland that year. He ignored the advice.

In previous years Shadow V had been watched to prevent a bomb being placed on it. For some reason in 1979, the year of Mountbatten’s death, that surveillance was withdrawn. A former military policeman, Graham Yuill, has since gone public saying that he was brought in to make a security assessment that summer. He reported a sighting of a known IRA activist in Mullaghmore and raised the question of security for Shadow V. Not only was nothing done about his report but he was immediately sent to Hong Kong and told never to mention his assignment.

• Read more: Opposing the royals: a brief history of republicanism in Britain

Mysteries remain. Given the enhanced threat, why wasn’t Mountbatten better protected in 1979? Could it be he didn’t want too much scrutiny? Or might the reason for his death have nothing to do with his symbolic royal status? Some people – myself not included – question whether he was really killed by the Provisional IRA, even though they were happy to take credit. An Irish career criminal named Patrick Holland claimed he was told by McMahon in Portlaoise Prison that he took the rap for Mountbatten’s murder to cover for others and that Mountbatten was actually killed by British Intelligence. Holland, who was planning a book on the Mountbatten murder, was found dead one morning in his prison cell. Aged 70, he was said to be suffering chest pains and doctors treated him the night before he died. Meanwhile Enoch Powell, citing a source in the Royal Ulster Constabulary, claimed the CIA were behind Mountbatten’s death because of his views on nuclear disarmament.

The background to Mountbatten’s death more than 40 years later continues to generate theories. Certainly there was either cock-up or conspiracy that he was not better protected in 1979. Meanwhile many of the government files which might provide insight and explanation remain closed under various Freedom of Information exemptions or have been destroyed.

Andrew Lownie is a literary agent and bestselling historian whose books include The Mountbattens: Their Lives and Loves (Blink Publishing, 2019). You can follow him on Twitter @andrewlownie