MI5 believed a hard core of around 750 teachers was trying to spread Communist propaganda in British schools after the Second World War, newly-released documents reveal.
A memo by the national security service written in August 1949 charts apparent Communist efforts to recruit teachers and infiltrate the leadership of the National Union of Teachers.
The document, which talks of a Soviet-backed drive dating back to 1920 to free worldwide education from capitalist "enslavement", also reports attempts to penetrate teaching in British colonies in the early post-war period.
According to MI5, which had heavily infiltrated the British Communist party in 1946, teachers accounted for 755 of its 38,766 known members.
By the time the report was written in August 1949, MI5 estimated that there had been "little change" with 700 to 750 staff in the party.
"Communist groups of teachers are active only in certain parts of the country," MI5 recorded. "These groups, where they do exist, aim at spreading communist propaganda in the schools among fellow teachers as well as the pupils."
The memo said that the party had "set much store" on efforts to infiltrate the NUT executive but had suffered a set-back two left-wing candidates doing poorly in 1949 union elections.
Steve Sinnott, the National Union of Teachers' general secretary and a former Communist Party member said: "The NUT has always been a broad church."