France had a bomb planted at its ambassador's London residence in an attempt to test British security arrangements, newly-released files show.
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 Mitterand.
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."
Other 1984 government records disclosed:
:: Mrs Thatcher made little effort to press for the release of Nelson Mandela during her first summit with South African prime minister PW Botha.
:: A faked recording in which President Ronald Reagan upbraids Mrs Thatcher over the Falkland War telling her to "control yourself" nearly sparked an international incident but was rumbled. The Government thought it could have been a Soviet Union plant.
:: Mrs Thatcher had 118 hair appointments during 1984 - including on five consecutive days during an economic summit in London in June.
:: Mrs Thatcher was unwilling to sanction the continuation of peace talks after the Brighton bombing.
:: British diplomats bet there would be no violence on the eve of the shooting of WPC Yvonne Fletcher despite two warnings from Libya and intelligence of possible "assassinations" at the London protests.
:: The Government hoped to stop miners smuggling in Soviet Union cash to support strikers by catching couriers with "a suitcase full of banknotes" coming through customs.
:: Mrs Thatcher secretly considered calling in the Army at the height of the 12-month miners' strike.
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