A telegraph machine from the Lusitania, thought to hold vital clues to its sinking after being torpedoed, has been lost during an unsupervised recovery mission, an Oireachtas committee has heard.

Questions have been raised as to why a diver was allowed to carry out the botched recovery without an archaeologist with him.

The Cunard British cruise liner, the largest ship in the world when built, was torpedoed by a German U-boat off the coast of Co Cork on 7 May 1915, with the loss of 1,201 lives.

Its wreck, 11 nautical miles off the Old Head of Kinsale, is regarded as a war grave and protected by an Underwater Heritage Order under National Monuments Acts.

Terry Allen of the National Monuments Service told the Joint Committee on Arts, Heritage, Regional, Rural and Gaeltacht Affairs that a telegraph from the wreck was lost during a dive on 13 July last year.

The operation was carried out by deep sea diver Eoin McGarry, on behalf of the US businessman Gregg Bemis, who owns the wreck.

Mr Allen said the telegraph was sent to the surface using a lift bag while the telegraph's pedestal was separately tied on to a shot line and hauled up.

The lift bag, which was tested prior to use and found to be in "perfect working order", had in fact been punctured with "a pin hole" which allowed the air within it to escape.

"Essentially the lift bag burst and the telegraph itself went back to the bottom," he said.

A number of attempts to locate it 90 metres below the surface have failed.

Mr Allen said the loss was "temporary hopefully and not permanent" but he warned of the threat from trawler nets dredged the sea bed.

He told the committee the loss would have happened with or without an archaeologist present, as is usually the case under regulations protecting the wreck.

Mr Allen told the committee the dive happened during "a one-off relaxation" of the requirement for an archaeologist.

The decision was made by himself "a day or two before the dive" because of "exceptional circumstances" including the bad weather which left just a small window for the operation.

Sinn Féin's Peadar Tóibín, chairman of the committee, said the Lusitania was one of the most important wrecks of Ireland and that the decision to allow a dive without supervision was a "significant break" in standard procedure.

Among the liner's 1,266 passengers and around 696 crew, there were 129 children, of whom 94 died as the ship, sailing from New York, sank in just 18 minutes.

The cause of a second explosion on the ship is still being investigated.

Built at the John Brown shipyard on the River Clyde in Scotland, the Lusitania was carrying 159 Americans, of whom 128 were killed.

The ship's captain, William Turner, who survived after the ship went down, had received messages on the morning of the disaster that there were German submarines in the area and he altered course.

Last October, during another dive a separate telegraph machine was recovered from the wreck, according to a spokeswoman for Minister for Heritage Heather Humphreys.

Mr Tóibín said the minister has "questions to answer" as to why there was a "clear break" in the strict oversight rules during the earlier botched recovery dive in May.

"It has come to my attention that an unsupervised dive was undertaken without archaeological methodology and was consented to by the Department (of Arts, Heritage, Regional, Rural and Gaeltacht Affairs) against the will of the National Museum of Ireland and the National Underwater Archaeology Unit," he added.

"This dive resulted in damage to an important historical and archaeological object and its subsequent loss.

"The minister has questions to answer."