Retired Warrant Officer Michael Norman, 62, formerly of the Coldstream Guards, was an anonymous witness at the Saville inquiry into the Bloody Sunday deaths in Londonderry in 1972, according to his former wife, Fiona McNab.

Scotland Yard confirmed that the death was being investigated as potentially suspicious.

Former Warrant Officer (Class 2) Norman, who left the Army in 1989 after 22 years, built a reputation in Northern Ireland and other operational theatres as a high-grade sniper and marksman. He is reputed to have shot dead up to six IRA gunmen during anti-terrorist operations. This was not confirmed by army sources.

Detective Inspector Stephen Leonard, of Hounslow police in West London, is leading inquiries. Scotland Yard said that the death had also been notified to an officer of commander rank because of the identity of the dead man. The police log makes reference to “underlying sensitive government issues”.

According to military sources, Mr Norman, who ended his career as a regimental quartermaster sergeant, was so highly regarded that he became a top instructor at Sandhurst.

He was found in a green BMW car on Sunday, April 17, in a road in Hounslow. A 9mm pistol was recovered. An inquest has been adjourned.

Ms McNab said she could not believe that he would have taken his own life. “He was a trained marksman. I can only believe this was some terrible accident if no one else was involved,” she said.

She said that Mr Norman had been accused of murdering a civilian, “believed to be an IRA operative, whom he shot at a roadblock during an anti-terrorist operation”. Mr Norman had been cleared.

“He was a fine man who served his country well.”

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