The Queen today hailed Britain's Second World War codebreakers whose vital work remained unknown for decades.
The Queen and the Duke of Edinburgh met veterans and unveiled a monument at Bletchley Park, Milton Keynes.
During the war Bletchley Park housed the government's secret Code and Cypher School, which obtained signals intelligence by breaking high-level encrypted enemy communications.
It was also home to the Colossus machine, the first programmable electronic computer.
In her speech the Queen said it was "impossible to overstate" the sense of gratitude to the men and women who worked there.
She said: "They were called to this place in the greatest secrecy, so much so that some of their families will never know the full extent of their contribution, as they set out on a seemingly impossible mission.
"We can be proud of the legacy of Bletchley, proud that Colossus was the first computer and that the British people, supported by our friends and allies, rose to the challenge."
Before her speech, the Queen had unveiled an 8ft memorial which carries the words We Also Served.
After the war the codebreakers who worked at Bletchley Park were instructed not to reveal any aspect of their mission.
Their decisive role in the Allied war effort, which some historians believe shortened the war by up to two years, was not fully recognised until the 1980s.
During her tour of the site the Queen stopped to talk with Bletchley Park veterans Oliver and Sheila Lawn, a married couple who met there during the war.
Mrs Lawn, 88, explained that the pair worked in different sections at the site but met during their leisure time.
She said: "We both joined one club and we were Scottish country dancing, and we met there. I saw this rather good dancer, we met and that was it."
After the war the couple took the government's instructions not to talk about what they had done at Bletchley Park to heart.
Mrs Lawn explained that she did not talk about codebreaking with her husband "until secrecy was lifted" decades later.
She said: "That was when the books started coming out in the 1980s.
"Then we talked a bit, not with great excitement, we just suddenly said 'oh yes, that's another subject of conversation'.
"We were told to forget because of secrecy, which was still vitally important.
"And you did forget, which I think the young people today think 'how did you manage to do that?', but that was the instruction."
Asked how she felt about the Queen's visit, Mrs Lawn said: "It was a very great honour and I'm very pleased that it happened because it's the culmination of a great deal of recent effort to save this place, it having been forgotten for all these years."
During their tour of Bletchley Park, the Queen and Philip were given demonstrations of encryption and decryption technology used by the Allies during the war.
In addition to the Colossus computer the royal couple were shown a reconstructed Turing Bombe Machine, an electromechanical device designed by English mathematician and computer scientist Alan Turing, who the Queen described in her speech as a "genius".
The Colossus and Bombe were used to break codes created by the German Enigma encryption and decryption machine, which is also on display at Bletchley Park.
The late Mr Turing's nephew Sir John Dermot Turing was among those present today.
Sir John, 50, who works as a solicitor, said his uncle died before he was born and described the mixed emotions he felt growing up in the shadow of his achievements.
He said: "In a way there's what you might describe as reflected glory. From time to time it can be quite burdensome.
"On the other hand, the Alan Turing story is so powerful, so well known and people are on the whole sympathetic to it that what you tend to find is an overwhelming sense of goodwill towards the whole tragic story."