MI5 believed that anyone thought to have links to the Communist party, however tenuous, were legitimate targets during the cold war, according to newly released files at the National Archives.

It also believed those groups or individuals thought to be potential targets of the Communist party should be closely watched.

David Ennals, a prominent figure in Britain’s United Nations Association who later became a Labour cabinet minister, and his brother, Martin, who became general secretary of the National Council for Civil Liberties and later secretary general of Amnesty International, were both monitored by MI5 and the police special branch, the files reveal.

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Ennals’ son, Sir Paul Ennals, told the Guardian: “It was hardly surprising that the secret services establishment found them all [there were three Ennals brothers, Martin, David, and John] of interest throughout their lives – their careers focused upon defending the rights of minority groups, setting up organisations to combat injustice, founding the Anti-Apartheid Movement and speaking out for what they believed.”

He added: “I don’t think such ideas and activities were extreme after the war, and they shouldn’t be now.”

MI5 justified its targeting of individuals and organisations, including the Anti-Apartheid Movement, the National Council for Civil Liberties, and CND, on the grounds either that some individual members were members of the Communist party, or that the party was suspected of trying to infiltrate them.

The files suggest that MI5, helped by the Metropolitan police special branch and customs officers – described as conducting “discreet” baggage searches – had an abundance of resources, given the number devoted to transcribing phone conversations by hand, opening letters and writing up reports of their targets’ movements.

While following a target remains MI5’s most labour-intensive task, developments in electronic eavesdropping have made the interception of communications much easier. Yet while during the cold war MI5 gleaned a mass of information from a single source – the communist party’s headquarters in King Street, Covent Garden – now it is faced with the prospect of intercepting many different, mobile, and encrypted sources of communications.

The targets in the files released on Friday were never threats to Britain’s national security, which MI5 was ordered to protect in 1989 at the very end of the cold war.

There are no clear guidelines on the release of MI5, MI6, or GCHQ files in the National Archives. Batches of files are released, apparently in an arbitrary manner with arbitrary cut-off dates. The files are carefully pored over, partly to conceal the names of informants, but also for other unexplained reasons.

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Under the 1958 Public Records Act, all government departments are supposed to preserve documents of historical interest, which should be handed to the National Archives at Kew or another suitable location once they are 30 years old.

An exception is made in under Section 3(4) of the act for those papers still being used, or which “ought to be retained for any other special reason” – a euphemism for national security reasons. Although the UK intelligence agencies are subject to the act, they have been granted blanket exemptions under Section 3(4), which are signed off once every 25 years, and they are also exempt from the Freedom of Information Act.

In the late 1990s, MI5 and GCHQ agreed that they would selectively start transferring records to Kew. MI6 says it cannot disclose any of its records, because of the need to protect the identities of its sources. As a consequence, the agency has disclosed none of the records that it has created or gathered since it was founded in 1909.