Robert Nairac, a British Army captain, was abducted by the IRA from a pub in County Armagh, Northern Ireland, in May 1977 while working undercover and attempting to gather intelligence.

The 28-year-old soldier was taken to a nearby forest across the border with the Republic of Ireland where he was tortured and shot dead.

Nairac is one of the highest profile of the so-called "Disappeared": 17 victims of paramilitary violence during the sectarian conflict over British rule in Northern Ireland known as "the Troubles", whose bodies were buried in secret.

Four, including Nairac's, are still missing.

Investigators with the Independent Commission for the Location of Victims' Remains (ICLVR) -- an agency tasked with locating the disappeared -- began the search in August on farmland adjacent to the border.

But it announced on Wednesday that it had been unsuccessful.

"It is bitterly disappointing that the search for Robert Nairac's remains has ended without success and our thoughts are with the Nairac family, in particular his sisters Rosemonde and Gabrielle," said a joint statement by Tim Dalton and Rosalie Flanagan of the ICLVR.

"The investigation and search team did everything they could to bring about a successful outcome but clearly more information is needed."

ICLVR's lead investigator Jon Hill appealed for further information to aid the search at the site in Faughart, around 90 kilometres (55 miles) north of Dublin.

"We were searching a relatively small area, less that one acre (0.4 hectares), and we did so because the information we had was credible," he added.

"It is our experience from other searches that while we have been in the right area the precise location has not been found first time round," he said.

UK Northern Ireland Secretary Hilary Benn said he was "saddened" by the unsuccessful search.

"My thoughts are with (Nairac's) sisters, who continue to live with the pain of not having the remains of their beloved brother returned to them," Benn said in a statement.