The 20 July plot was masterminded by Claus von Stauffenberg, who planted bomb in Hitler's Wolf's Lair

The plot failed when the bomb was moved behind a thick table leg, shielding Hitler from the blast 

Co-conspirator, lawyer Otto John, was known to have been involved and fled afterwards to Madrid

But a file which was found by historian Nigel West reveals he was working for MI6 for two years in lead-up

He had the codename of 'Whisky' and met with handlers 12 times before the plot was carried out