Steve Cardownie, now deputy leader of Edinburgh City Council, was the subject of acid criticism in a confidential memo written by Sir William Kerr Fraser, then permanent secretary at the Scottish Office.
As chairman of the Civil and Public Servants Association, Mr Cardownie met Sir William to discuss the impact of Margaret Thatcher's Tory government banning unions from GCHQ, the security service centre in Cheltenham.
The move provoked protests from unions fearing similar bans elsewhere.
In 1984, two days after the ban was announced, Mr Cardownie and six other union leaders took their grievance to Sir William.
The official note said Mr Cardownie rejected the "offensive" inference that union membership threatened national security, claimed it was part of a government effort to break the trade union movement, and raised the possibility of strike action.
Four days later, Sir William wrote a memo marked "confidential" to the then Scottish Secretary, Sir George Younger, saying the two "principal spokesmen for the unions delivered largely political harangues". One was Mr Cardownie.
Sir William was reluctant for Sir George to face Mr Cardownie, but warned refusal to meet unions could spark a strike.
In the memo, released by the National Records of Scotland, Sir William said union militants in the Scottish Office had found it hard to find issues to justify industrial action.
"Suddenly they have been presented with an issue which they may well be able to exploit for this purpose," he wrote. "A refusal to meet them would provide the militants with another goad with which to prod their members."
Younger met the unions 10 days later.
Mr Cardownie said: "I was robust in defending the membership. I had a mandate to be militant."