Luckily for the MoD, the stolen plans were from 1943 and the target was occupied France during the second world war

Amid the mayhem when the British embassy in Tehran was stormed last Tuesday, a mob destroyed antique oil portraits of Queen Victoria and Edward VII, made off with a poster from the film, Pulp Fiction, but narrowly missed dog-napping the ambassador's terrier, Pumpkin.

However, the Basij commanders and Revolutionary Guard (IRGC) officers said by western diplomats to be in the crowd, must have thought they had hit the jackpot when the raiders came across embassy documents in the ambassador's office marked "most secret" in red ink, giving details of a gigantic invasion plan.

The intelligence coup is diminished however by the fact that the plan in question was an advance peek at Operation Overlord, the D-Day landings, which Winston Churchill and Franklin D Roosevelt provided to Stalin at the 1943 Tehran conference, where the western powers agreed to accept the Soviet domination of eastern Europe.

The British ambassador, Dominick Chilcott had taken the minutes of the Tehran meeting out of the embassy safe on the occasion of a dinner to commemorate its 68th anniversary.

"We're hoping it takes the IRGC translators a good long time to realise the plans are 70 years old and talking about the invasion of France," a British diplomat said.

Some other Tehran conference memorabilia was salvaged, including a photo album of the event. But that, combined with the hope of causing temporary confusion to Iranian intelligence analysts, is small comfort for the UK foreign office amid the devastation of a venerable, antique-filled embassy.