By Gordon Corera
Security correspondent, BBC News
Stephen De Mowbray joined the Secret Service after World War II
For 30 years Stephen De Mowbray has maintained a self-imposed silence on a career that once took him to the heart of one of British intelligence's most controversial episodes.
In 1979 he quit his job with the Secret Service because he believed officials had failed to take seriously the claim that British intelligence had been further penetrated by its enemy - the Soviet Union's KGB.
A number of spies had been discovered in the 1960s but De Mowbray believed there were more. But he found no-one at the top willing to listen.
"People thought I was either mad or bad because I was trying to do something," he says of that time.
Three decades later, De Mowbray decided to tell his side of the story after reading the authorised history of the Security Service (MI5), published last October.
It dismisses the view that there were further traitors in the Security Service.
In the book, De Mowbray's claims are the subject of a chapter subtitled "paranoid tendencies" which recounts his work as well as that of two colleagues, Peter Wright (author of the controversial Spycatcher) and Arthur Martin.
The book quotes an MI5 director saying of the group: "Involvement in counter-espionage cases induces in some a form of paranoia."
De Mowbray himself is referred to - although not by name - as "the leading SIS (Secret Intelligence Service) conspiracy theorist".
"I was this SIS officer," De Mowbray confirms.
De Mowbray joined the Secret Service shortly after World War II and in the 1960s was assigned to work in the field of Soviet counter-intelligence investigating the operations of the KGB.
The British establishment was in the process of being rocked by a series of scandals in which a number of individuals were revealed to be working for the other side.
De Mowbray was assigned to work on the case of a KGB officer named Anatoliy Golitsyn, who defected in 1961.
Golitsyn remains a controversial figure. De Mowbray argues he provided a number of crucial leads. Critics say he became prone to exaggeration.
Golitsyn's information suggested there were more traitors in the West, including within its intelligence agencies.
At the same time, two MI5 officers - Arthur Martin and Peter Wright - had also both come separately to the same conclusion - that there was a penetration at the highest reaches of the Security Service.
They called on MI6 to help and De Mowbray was assigned to assist them.
"There were extraordinary things going on," recalls De Mowbray.
"Martin was running people against the Soviets and those operations were going wonky."
Meanwhile Peter Wright's bugging devices, which had been installed in Soviet premises around the world, were also failing to produce intelligence.
These operations were known only to very few senior officers in MI5.
"I was utterly horrified at the thought that this was happening," says De Mowbray.
De Mowbray still believes he was right
When the small group added in Golitsyn's claims they came to believe that there was a mole at the very top - either Graham Mitchell, the number two at MI5, or his boss Roger Hollis.
"I vowed to myself that I would never let go of this case," recalls De Mowbray.
In his authorised history of MI5, Christopher Andrew describes the investigations into Hollis and Mitchell as "the most traumatic episodes in the Cold War history of the Security Service".
Mitchell was investigated first. As recounted in the authorised history, this involved bugging his phone, feeding him false information and putting him under close surveillance.
"We followed Mitchell all over the place, downtown when he left from the office, trying to chase him up the steps in Waterloo when he went home," recalls De Mowbray.
Even after his retirement, Mitchell was still monitored. Nothing was found. Next Hollis was investigated but eventually also cleared.
"There were suspicions with both of them," De Mowbray argues. "There are not suspicions now. But somebody was doing it."
In 1964, De Mowbray was posted to Washington where he worked more closely with Golitsyn and his sponsor in the CIA, James Jesus Angleton.
Angleton became convinced that the KGB was mounting a wide-scale deception campaign to hide its true capabilities and the presence of its spies in the West.
He was eventually dismissed from the CIA. Critics said he damaged the organisation through his investigations into a CIA "mole" who never existed.
In the authorised history of MI5, it is argued that Golitsyn became an increasing "liability" because of his "passionately paranoid tendencies".
De Mowbray disagrees with the portrayal of Golitsyn. He says he has been misrepresented and disputes details presented of Golitsyn's visits to the UK, arguing that some of them were genuinely productive in terms of intelligence leads.
De Mowbray became increasingly frustrated at the lack of action and complained repeatedly to his superiors through the 1970s.
He was moved away from the investigation. "I could not reconcile myself to doing nothing: I had made so many commitments to myself and to others to pursue the problem to the end that I could not wash my hands and forget about it," he explains.
He argued that MI5 had not properly investigated itself and was incapable of doing so. "It was a very difficult situation for years on end," he says now of that time.
Peter Wright was also convinced of high-level penetration
De Mowbray went as far as approaching the Cabinet Secretary, Sir John (later Lord) Hunt. He referred De Mowbray on to a former Cabinet Secretary, Lord Trend, who conducted a review of the subject and found insufficient evidence to support the allegations.
"Don't expect me to tear Whitehall apart about all this," De Mowbray recalls Lord Trend telling him.
He was told he could not have his old job back in counter-intelligence and soon after De Mowbray applied for early retirement.
He went off to the US initially to help Golitsyn write a book on Soviet deception and later to help him on his unpublished memoirs. He had no further contact with the intelligence services and steered clear of public comment until reading the authorised history of MI5.
The consensus view has now developed, reflected in Christopher Andrew's book, that there were no further high-level penetrations in British intelligence.
But De Mowbray remains convinced that there is a dark secret that has still not come out.
"When I left most people were oblivious of the situation", he says. "Maybe I was wrong? But I don't think I was."