<On one occasion de Mowbray heard that Mitchell, an ardent chess fan, was attending a tournament in Eastbourne in which Russians were taking part. De Mowbray commandeered an MI6 colleague with a fast sports car to whisk him down to the tournament, but without any results.
Another time, de Mowbray was following Mitchell through a rush-hour crowd in London when the MI5 officer stopped, turned and looked straight at him. Mitchell said nothing, but stared into de Mowbray’s face for several seconds before turning on his heels and walking away. He knew he was being watched.
In one of the most bizarre episodes in the history of British intelligence, three female employees of MI6 took shifts spying through a peephole drilled through Mitchell’s office wall. A secret camera recorded the changes in his body language, his eyes sinking into black hollows as he spiralled into depression as a result of his awareness that he was under suspicion.
Mitchell took early retirement, but even after his departure he was kept under surveillance, although de Mowbray and Wright remained convinced that Hollis was the real Soviet mole.
Fuelling the suspicions over Hollis was the fact that he had spent part of the 1930s in China, as a representative of British American Tobacco, associating with a number of communists, including the Soviet spies Richard Sorge and Agnes Smedley, before heading MI5’s anti-Soviet section during a period when the Cambridge Spy Ring were all active.
Eventually the CIA were told that Hollis had been cleared, so de Mowbray appealed first to Sir John Rennie, who took over as chief of MI6 in 1968. When Rennie declined to do anything, de Mowbray tried to speak to the prime minister Harold Wilson.
He did not get to speak to Wilson but had an interview with Sir John Hunt, the Cabinet Secretary, who initially thought he was mad. Hunt contacted Sir Dick White, now retired, and asked him if de Mowbray was “a screwball”.
White replied that de Mowbray was “patriotic, hardworking and obsessed”. White also refused to rule out that Hollis was the mole. Hunt asked his predecessor, Lord Trend, to carry out an inquiry. Trend spoke to de Mowbray, warning him that he was not going “to tear Whitehall apart about all this”.
Trend’s findings remain classified but its conclusion was ultimately that there was not enough evidence either to clear or condemn Hollis. De Mowbray eventually resigned, furious that no one seemed prepared to do anything about hostile penetration of MI5.
When Professor Christopher Andrew published the authorised history of MI5 in 2009, in which he dismissed de Mowbray as one of a trio, with Martin and Wright, of conspiracy theorists with “paranoid tendencies”, de Mowbray felt compelled to speak out, having not breathed a word in public about it for 30 years.
He told Gordon Corera (author of The Art of Betrayal: The Secret History of MI6, 2012) that when he left MI6 no one seemed willing to countenance the idea of further Soviet penetration of the top of the Security Service. But he remained convinced that he was right.>