According to the Royal Household:
The Queen visited MI5, the UK’s domestic counter-intelligence and security agency.
[She said] “Because of the nature of your work, it is without public recognition, so it is on behalf of the country that I say to you all, thank you.”
From convincing Hitler the D-Day landings were going to be at Calais rather than Normandy, to misreporting where German rockets were landing so they would adjust their targeting further away from populated areas – their work proved invaluable to the allied war effort.
To read the entire article from the Royal Household, please click here.