A French secret agent planted a “bomb” in the grounds of the residence of Paris’s envoy to London to test British security – provoking fury when it was discovered by Scotland Yard.

Two containers of high explosive were discovered by a sniffer dog shortly before the ambassador was due to host President François Mitterrand in October 1984. When police questioned the security officer responsible, more explosive was found stashed in his hotel room.

Cabinet minutes show that ministers, who had recently survived the Brighton bomb, were unimpressed by the failure of Paris to offer an explanation for the incident, describing it as “inexplicable and unacceptable”.

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