France had a bomb planted at its ambassador's London residence in an attempt to test British security arrangements, newly-released files show, reports Sky News.
The 1984 plot left police "extremely annoyed" after they discovered two containers of high explosives stashed in the grounds ahead of a visit by then president, Francois Mitterrand.
According to the documents unveiled by the National Archives, at Kew in southwest London, the French security officer who planted the bomb was later found with further explosives in his room at the Grosvenor House Hotel.
Ministers in Margaret Thatcher's government slammed the French refusal to offer an adequate explanation for the October 1984 debacle as "inexplicable and unacceptable", Cabinet minutes showed.
The minutes noted that the ministers demanded the exercise would not be repeated by the French, saying: "In particular assurances should be sought regarding security at Buckingham Palace.
"The police were naturally extremely annoyed at what had occurred."
Read more of this report from Sky News.