The Duke of Edinburgh has remarked how insecure wartime communication was after reading the translation of a call between Winston Churchill and Franklin Roosevelt which was intercepted by German spies.
GCHQ historian and former intelligence officer Tony Comer told the Duke that the highly sensitive document showed that the Italians were secretly negotiating with the Allies to switch sides, as the Duke remarked: "So it wasn't quite so secure as we thought."
The Queen and Prince Philip were shown the translation of the call between the two leaders from 1943 during a visit to mark the official opening of the National Cyber Security Centre (NCSC) in London.
German intelligence services were able to intercept the communication due to the strength of the signal needed to boost the telephone exchange across the Atlantic Ocean, as Allied leaders discussed wartime strategy.
Among the other items viewed by the Royal couple was a selection of historic code-breaking devices, including an 1890s naval code wheel and a 1943 Telephone Privacy Directory with an extension number for Buckingham Palace.
Mr Comer said a scrambling device enabled secure communication across a telephone network, before being decoded by a separate scrambler at the other end.
In response to Mr Comer's explanation that the device was secure "for its time", the Duke quipped:
"Well, it didn't last very long."
NCSC chief executive Ciaran Martin later revealed in a speech to mark the Queen's visit that the Duke had urged him to recruit more people who "do not pre-date the internet".
Mr Martin said: "On the way out, and in a quote with his permission, the Duke of Edinburgh did ask: 'When are you all going back to work?'"
Among those who spoke to the Queen was 12-year-old Cerys Ives, from Birstall, Leicestershire, who has been selected by NCSC officials to take part in the CyberFirst initiative, aimed at getting young people into cyber security.
Miss Ives was spotted after she sent an innovative message to the centre, asking if she could apply.
"I sent them an encrypted email asking about the CyberFirst competition for 13 to 15- year-olds, because I was only 12 at the time.
"It took about four days for them to decrypt it and send a reply."
Cerys said despite the four day wait for a message back from the centre, it took just half an hour to decrypt the encoded return message, which read: "You inspire us more than we inspire you."