British diplomats complained bitterly in private about the lack of security provided by the Irish authorities for Lord Mountbatten following his assassination by the IRA.
Mountbatten was killed on August 27, 1979, when the IRA detonated a bomb on his boat at his holiday home in Mullaghmore, Co Sligo.
Three other people died in the explosion.
The information, reported in today's Irish Independent, has been released by the British National Archives at Kew under the 30-year rule.
It shows that in a telegram to Foreign Secretary Lord Carrington the British ambassador in Dublin, Robin Haydon, said there was a widespread belief the attack on Lord Mountbatten could have been prevented.
He said the concerns about the level of security provided by the Garda Siochana were shared by local people who had "greatly liked and respected" the peer and had the Garda Siochana been more vigilant and conscientious, the murders might not have happened.
Mr Haydon said he had been told by Lord Mountbatten's daughter, Lady Pamela Hicks, that it was the first year when the boat did not have a police guard on it during the day.
He said he found it extraordinary that the boat was apparently not searched by the garda before it sailed.