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.

He was also said to have bankrolled the scab Union of Democratic Mineworkers and organised the legal action that led to the strike being ruled unlawful.

He bragged that Thatcher came to rely on him totally, claiming: “It got to the point where she really let me run it.”

In February 1985 the PM’s political secretary Stephen Sherbourne wrote to advise her that Mr Hart had begun to pursue his own agenda, briefing against ministers including then energy secretary Peter Walker.

“Though DH has on occasions provided you with useful intelligence he has recently been pursuing his own ends at the expense of those of the government,” he wrote.

“DH has his own views on how the coal strike should end and has been pursuing his cause even when it conflicted with the interests of yourself and Peter Walker. And in so doing he has exploited his No 10 connection.”