The British Foreign Office feared that the discovery by divers of explosives in a 1982 salvage operation on board HMS Lusitania, sunk off Kinsale by a German U-Boat in 1915, would be used against it by the Irish government in the United States, according to newly released British files.

The sinking of the liner helped to sway American opinion behind entry into the first World War, but the Germans insisted in the aftermath that the British had “tried to use the lives” of American passengers to cover the transport of munitions.

However, the British, apart from accepting that it had carried 5,000 boxes of small arms cartridges, have always insisted that it was carrying no other munitions - a belief they repeatedly declared in letters newly released by the UK National Archive in London.

“If it were now to come to light that there was after all some justification, however slight, for the torpedoing, HMG’s relations with America could well suffer”, British Treasury official Jim Coombes told the Foreign Office’s North American Department in August 1982.

Writing just months after Anglo-Irish relations had deteriorated over the Falklands, Mr Coombes went on: “Your Republic of Ireland Desk is of the opinion that the Irish would seek to create as much uproar as possible over such a discovery.”

The issue arose 67 years after the sinking when the Treasury was told that salvagers were about to begin work on the Lusitania, which lies 11 miles off the Old Head of Kinsale in 300 feet of water.

Despite denying that heavy munitions had been carried, the Ministry of Defence warned salvagers of “the obvious dangers from the presence of known small arms cartridges”, and also “from other explosives if they happened to be present”.

In a letter, salvagers were told that “the only explosives” the Ministry of Defence knew about were the 5,000 cases of small-arms cartridges, “but it would be imprudent not to point out the obvious but real danger inherent if explosives did happen to be present.

“In that unlikely event, you are strongly advised to stop operations and consider your position most carefully as there would be real danger to life and limb,” the ministry told the Salvage Association, in a letter dated August 3rd, 1982.

Small arms cartridges were found by the Co Waterford-based diver Eoin McGarry, on behalf of the Lusitania's American owner, Gregg Bemis, in September 2008 in a part of the sunken liner where ammunition was never previously known to have been stored for the voyage.