Margaret Thatcher’s government drew up a list of “subversive” civil servants who were to be closely monitored or blocked from potential promotion.
According to National Archives files, security services monitored 1,420 Whitehall staff who were to be kept away from computers and revenue collection roles.
The list included 733 people who were identified as Trotskyists and a further 607 as communists.
Forty-five were said to be fascists, while 35 were down as Welsh or Scottish nationalists, “black or Asian racial extremists” or anarchists.
MI5 also held similar lists of suspect local councillors and active trade unionists.
They were unable to do the same with NHS workers “without alerting those concerned, with a high risk of public exposure of our investigation”, but did monitor teachers by arranging for school inspectors to report directly to security service officials.
In 1985, MI5 reportedly identified 50,000 people across Britain who it described as “subversive”.
It prompted the revival of the 1970s Whitehall body known as the Inter-departmental Group on Subversion in Public Life (SPL), which brought together MI5, police and government departments.
It defined subversion as “activities which threaten the wellbeing of the state and are intended to undermine or overthrow parliamentary democracy by political, industrial or violent means”.
The shift came after a major strike the previous year of health department computer operators in Newcastle saw pension and child benefit payments to millions of people delayed.
Senior civil servants blamed three key organisers of the strike who appeared to be members of the Trotskyist Militant Tendency movement.
The Cabinet Office told the Guardian it regarded the investigation as a historical matter on which it did not wish to comment.