Tory councillors on the former Glasgow Corporation sent an urgent telegram to Edward Heath, the British Prime Minister in 1973, asking him to overrule the Scottish Secretary’s decision to allow five selective schools in Glasgow to be turned comprehensive and a sixth to be closed.

Scottish Office papers dating from 30 years ago show that the Conservative group on the council, led by Walter Wober, told Mr Heath that they no longer had confidence in Gordon Campbell, the then Scottish Secretary of State, and felt he had deserted his own stated requirement of consultations with parents over the Labour-led council plan to make all secondary schools comprehensive.

The Tory councillors’ telegram added: “To support Labour Party dogma in denying parents the freedom of choice in education seriously threatens credibility and support.”

But their pleas for the PM to intervene fell on deaf ears and Mr Campbell retained Downing Street’s support. He insisted that his role was that of a “lightning conductor who can usefully earth the shafts of criticism and can settle matters without storms of controversy”.

In the case of the row over plans to end the selective status of schools such as the High School of Glasgow, Allan Glen’s School and Hillhead High School, he continued to argue that there were no educational reasons for opposing the move, although his office called for details from the office of the then Education Secretary, Margaret Thatcher, of why she was prepared to stop plans by Birmingham City Council for 60 of its schools, including 18 selective schools, which it planned to turn into comprehensives.

Correspondence between Douglas Hurd, Ted Heath’s then political secretary, and Campbell showed the political pressure to “cope with the Birmingham argument” which Scottish Conservative MPs were advancing as they pressed the case for intervention by the Tory Government to prevent the march of Labour-controlled councils’ towards comprehensivisation of the education system.

While it was clear that the English education legislation gave stronger powers of intervention to politicians than the equivalent Scottish legislation, the archive papers show that another force was at play: local government reorganisation was looming.

As a memo by one senior civil servant said: “A decision to reject Edinburgh and Glasgow’s proposals for reorganisation of secondary education other than on educational grounds would scarcely have been consistent with his action in promoting the current Bill for reorganisation of local government which is designed to strengthen local government and to give local authorities considerably increased powers to reach their own decisions.”

It also emerged that pupils at the High School of Glasgow might be forced to go to further education colleges if they wished to pursue all their subjects once their school closed because other schools might not be able to offer the same wide-ranging curriculum. R S Johnston, HMI, pointed out that several pupils already attended “twilight classes” at a central point because their own school was not able to offer Higher-level classes for small groups and because of staff shortages.

To a parent who complained that his child would not be able to study eight or nine Highers in one year at any other school once the High School closed, he replied: “Glasgow will be encouraging schools not to present pupils in more than five to six Highers in fifth year. Even with a 45-period week, if a boy is being prepared for eight or nine Highers at one sitting, the examination emphasis is crushing and it is perhaps not surprising that no other school provides such a diet.”