In one of his last interviews Dr Millward explained how he warmed to the then 20-year-old prince during their hour-long lectures at Aberystwyth University.
“I was not in favour of him becoming the Prince of Wales but I didn't argue the point with him," he said.
“I got on quite well with him, he was good to get to know.
“I found him intelligent and quite charming, we stayed in touch for a long time afterwards.”
The ex-politician said he was surprised when he was asked to teach Charles, as he had come close to being arrested during a Welsh language protest in 1963, six years before the Prince's controversial investiture.
However, he tried to treat Charles as he would any other student.
“When I met him I tried to be neutral, I just treated him the same as any other student.
“I’m not particularly proud of being his Welsh teacher, it was forced upon me.”
Prince Charles stayed in touch with Dr Millward, who would give him advice if he had to make an address in Welsh during his engagements in Wales.