A French secret agent had a bomb planted at the residence of Paris' envoy to London in an attempt to test British security arrangements.
According to the documents released by the National Archives, two containers of high explosive were discovered by a sniffer dog shortly before the ambassador was due to host then President Francois Mitterrand in October 1984.
When police questioned the security officer responsible, more explosive was found in his hotel room, the Independent reports.
Ministers in Margaret Thatcher's government slammed the French refusal to offer an adequate explanation for the debacle as 'inexplicable and unacceptable,' the report added.