Counter Subversion Branch

In Dick White's 1953 re-organisation, F Branch had responsibility for counter-subversion at home, while E Branch was responsible for counter-subversion in the British Empire and Commonwealth.^[3]

In 1972, F Branch director John Jones defined subversion as "activities threatening the safety or well-being of the State and intended to undermine of overthrow Parliamentary democracy by political, industrial or violent means."^[4]

Peter Wright describes a meeting of senior A Branch and MI5 F Branch officers to discuss the "far and wide left" early in Michael Hanley's tenure as head of MI5:

The Prime Minister and Home Secretary had left him in no doubt that they wanted a major increase in effort on this target. He then handed over to a young and ambitious F Branch officer, David Ransome, who outlined the activities and structure of a host of left-wing splinter groups , like the Workers Revolutionary Party (WRP) and the Socialist Workers' Party (SWP).^[5]

Stephen Dorril states that Ranson was an F2 officer at the time of this meeting, which he places in 1972.^[6]

In 1976, Director General Michael Hanley established a separate FX branch to deal with Irish terrorism. Its director continued to report to the director of F Branch.^[7]

By the time former F Branch director John Jones became head of MI5 in 1981, it had become the premier branch of the service, largely at the expense of K Branch counter-espionage.^[8]

In 1988, the FX Branch was renamed G Branch.^[9] The rest of F Branch was incorporated into MI5 C Branch at around the same time.^[10]