Positive discrimination was mooted by the Scottish Office in 1973 amid fears that the supply of Catholic trainee teachers could dry up, according to Scottish papers released this week under the 30-year rule.
At the time, Scottish education minister Hector Monro was embroiled in a row over Roman Catholic recruitment at Dunfermline College of Physical Education, now part of Edinburgh University.
An aggrieved Catholic man complained that his daughter had failed to win a place at the college for the second year running, despite the grave shortage of PE teachers in Catholic schools. He demanded to know what the entrance criteria were and what proportion of entrants were Catholics.
Dissatisfied with the college's response that religion played no part in its selection process, the aggrieved father tackled the Scottish Office.
The department's subsequent investigation uncovered a "likely deterioration" in the supply of Catholic teachers. It found that, while some 20 per cent of Scottish pupils were educated in Catholic schools, only 13 per cent of secondary teacher training entrants and 9 per cent of BEd entrants were Catholics.
An internal Scottish Office memo warned that, given the keen competition for places at Dunfermline, the proportion of Catholic entrants risked falling further if exam results in Catholic schools did not keep pace with those in non-denominational schools.
The memo suggested that if non-denominational schools had enough PE teachers, "it would be possible (but arguably not desirable) to bias admission in favour of Roman Catholic applicants". But another memo advised Mr Monro to avoid this controversial area when replying to the aggrieved father.