More than 1,000 civilians – including 128 Americans – died when the liner was torpedoed by a German submarine eight miles off the coast of Ireland on May 7, 1915.

British propagandists exploited the attack, arguing it was an unprovoked assault on a civilian ship, and it helped swing popular opinion in the United States against Germany - paving the way for America's entry into the conflict two years later.

However newly released documents show that in 1982 the government feared attempts to survey the wreck would reveal an illegal stash of ammunition, which almost 70 years on, would discredit the premise on which the Americans were drawn into the war.

The files show that the Ministry of Defence even went as far as warning divers planning to explore the wreck that it could contain explosives - even though it had always maintained the ship was not carrying any on her final, fateful voyage....

The Foreign and Commonwealth Office files, released by the National Archives today under the 30-year rule, disclose that officials feared the controversy would “literally blow up on us”.