Thatcher was warned to break off relations with a self-serving millionaire who trumpeted himself as the architect of the miners’ defeat in 1984-85, papers released under the 30-year rule show.

Officials feared that property developer David Hart was exploiting his connections with No 10 for his own ends, warning the Tory prime minister that he would end up causing her “grave embarrassment.”

Hart established himself as a go-between between Thatcher and National Coal Board chairman Ian MacGregor.