The detectives will meet the former officers at various locations in England and Scotland, including Hereford, the headquarters of the SAS.

Among those expected to be interviewed is General Sir Michael Rose, who commanded the SAS at the Iranian embassy siege in London in 1980, and during the 1982 Falklands war.

Gen Rose, who commanded G Squadron of the SAS when Nairac was operating in south Armagh, eventually became Adjutant General, the No2 in the British Army before retiring.

Another officer to be questioned is Col Clive Fairweather, who was at one time Gen Rose's second-in-command of the SAS, and who was Nairac's boss when he was kidnapped and murdered by an IRA gang.

The PSNI Major Incident Team recently took over investigation from the Historical Inquiries Team, which had already interviewed several people involved with the Grenadier Guards officer.

Captain Nairac, who was posing as a member of the Official IRA, was abducted from a pub in Drumintee, south Armagh, in May 1977, by at least seven men and severely beaten before being driven across the Irish border and beaten again. He was then murdered.

Two years after his death he was awarded a posthumous George Cross, second only to Britain's highest gallantry decoration, the Victoria Cross.

It is believed the Queen herself demanded Nairac's gallantry be recognised.

At the time of his death he was working as a military intelligence liaison officer with the army brigade responsible for security in south Armagh, liaising with the Army, police and special forces units operating in the area.

Last week, 57-year-old Kevin Crilly from Jonesborough in Co Armagh appeared in court in Newry on charges connected with Capt Nairac's disappearance.

The police are also trying to extradite two other men from America whom they believe can help with their inquiries.

One detective said: "We are having to look at this whole murder with fresh eyes.

"We have greater resources and manpower than the HIT and we have a senior officer in charge who has had great success recently re-investigating old crimes, including murder.

"There are several people no longer around who thought they had got away with murder and other serious crimes until we caught up with them.

"We need to talk to these SAS people because we need to know exactly what was happening on the ground in that very dangerous area back then.

He added: "We shall re-visit their statements and re-interview them in case anything new surfaces.

"There have been many changes to investigations and the way evidence is gathered and has become admissible in the years since Capt Nairac's murder."

Although Capt Nairac's parents have died, the detectives have been keeping the murdered officer's two sisters up to date with their investigation.

One of their aims is to try to discover where the IRA gang buried his body. It is believed to have been placed in a shallow grave, then moved to another location near the border.

It is understood that former IRA men are to be interviewed and if any clues on the whereabouts of the body are unearthed, a search will be ordered.

One of the former SAS officers on the police list said yesterday: "I am sure we will all do everything we can to help the police.

"Neither we, nor the regiment, has anything to hide. Although he was not a member of the SAS he worked with us and we all feel it is time his body was returned to his family for a Christian burial.

"Robert faced a terribly brutal death and if there are more people out there who took part in this then it is our duty to help the police bring them to justice before the courts."

The police are also believed to be planning to meet former members of the shadowy 14 Intelligence Company, the 'hush-hush' unit which shadowed many terrorists with whom Nairac also had close links.

A former commanding officer of the unit said: "There have been suggestions that he was one of ours, or in the SAS. He was in neither, but we will help the police in any way we can."