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Dozens of "subversive" Welsh nationalists were secretly blacklisted under Margaret Thatcher's government, newly released documents reveal.

Papers released by the National Archives show civil servants thought to be subversive were monitored and in some cases blocked from promotions and kept away from computers.

A previously secret document from 1985 also claimed that Welsh national extremists posed a problem due to their "propensity for violence".

Working with MI5, the government found 1,420 civil servants who would be closely watched.

Of these, 35 were claimed to be Welsh or Scottish nationalists.

A document from December 1984 also stated there were up to 50 Welsh nationalist extremists across the UK part of subversive groups.

In 1985, it was estimated there were around 50,000 people across the UK that MI5 would describe as subversive.

In the documents, "subversive activities" are defined as "those which threaten the safety or well-being of the State and are intended to undermine or overthrow parliamentary democracy by political, industrial or violent means.”

(Image: WalesOnline)

The blacklist was drawn up in 1985 after a seven-month long strike in Newcastle was found to have been orchestrated by "subversives" working for the civil service. That strike was by the computer operators of the Department of Health and Social Services in Newcastle.

The information was presented to the 'Inter-departmental Group on Subversion in Public Life', known as SPL.

The body was originally created in the 1970s but was brought back during the miners' strike.

A “very small” number of subversives working in the civil service were thought to be “active troublemakers” who would obstruct work or create difficulties.

Recommended counter-measures at the time included all departments in the civil service to review regularly with MI5 their lists of “known subversives”.

This was encouraged so top government officials could be kept up to date with the identity and location of subversives to monitor the threat they posed.

The exact figures for the number of Welsh nationalists on the blacklist is not revealed in the papers as they are grouped with Scottish nationalists.

One document from 1985 said: "Members of some anarchist organisations, and of other groups, such as certain black racial extremists and London, and Scottish and Welsh nationalist extremists, also present localised or limited public order problems largely resulting from their propensity for violence."

Only the largest Communist and Trotskyist parties were judged to pose a significant threat on a national scale.

The secrecy of the operation was stressed in the documents, with its existence kept to a "very restricted circle".

Thatcher was not shown the SPL reports detailing subversive numbers, but said in 1985 that managers should be "very ready to sack subversive troublemakers if they showed any cause under the civil service rules".

A Cabinet Office spokesperson said: "The Inter-departmental Group on Subversion in Public Life (SPL) is no longer in operation and there is no other unit conducting similar work."