For years, I suspected MI5 interfered in the miners’ strike. The truth was even more shocking than I thought | Mark Hollingsworth

A document buried in the National Archives reveals how the security service abused its power to help the government win

MI5 has always carefully guarded its secrets. In her memoir, the ironically titled Open Secret, the former head of the security service Stella Rimington assessed the role of MI5 during the 1984-85 miners' strike: We limited our investigations to the activities of those who were using the strike for subversive purposes. The reports we issued to Whitehall during that time were most carefully scrutinised to ensure they referred only to matters properly within our remit."

This was the official version of events. Yet buried in the National Archives at Kew is a secret document that casts new light on Rimington's account of MI5's role during an industrial dispute that represented the most serious challenge to Margaret Thatcher's premiership. At a moment in which we are commemorating the 40th anniversary of the miners' strikes, it is very much worth looking at this document closely.

Mark Hollingsworth is a freelance journalist and the author of books including Defending the Realm, Londongrad: From Russia with Cash and Agents of Influence: How the KGB Subverted Western Democracies.