Media playback is unsupported on your device Media caption The Queen pays tribute to the codebreakers who worked at the top secret Bletchley Park and broke the German Enigma codes in WWII.

The Queen has paid tribute to the codebreakers who worked at Bletchley Park, the secret cypher base in WWII which broke the German Enigma codes.

Image caption Bletchley Park houses the National Code Centre and the National Museum of Computing

She said it was impossible to over-state the "deep sense of admiration, gratitude and national debt" owed to such a "remarkable group of people".

The Queen unveiled a memorial to remember those codebreakers who have died.

Some of the surviving codebreakers were present for the event.

Historians estimate that breakthroughs at Bletchley shortened the war by two years.

Though the role codebreaking played in the war is now widely celebrated in films such as Enigma, for 30 years after the end of the war Bletchley Park's role remained a secret.

Analysis Its existence was one of the most closely guarded secrets in WWII; its contribution to the Allied victory in that conflict was such that one historian has said that without it, the war would have lasted for at least two more years. It was at Bletchley Park that some of Britain's best crossword experts, mathematicians and chess champions wrestled with the supposedly unbreakable German radio messages sent with their Enigma encryption machines. That they did break the Enigma codes made an incalculable contribution to the battle of the Atlantic, allowing the movement of U-Boats to be discovered

Simon Greenish, director of the Bletchley Park Trust, said: "Nobody knew what had gone on at Bletchley Park for in excess of 30 years after the war, and only then the stories started to come out. So I think there's certainly an argument that the codebreakers are being recognised late in the day for what they did.

"There is no doubt that it shortened the war by several years and saved many many lives in the process," he said.

"Some historians are beginning to think that perhaps it was the bit that made the difference."

During her visit, the Queen saw the restoration of a Colossus machine and an Enigma display, before seeing a rebuilt working Turing Bombe machine which was used to crack the codes.

More than 9,000 staff worked at the Government Code and Cypher school, as Bletchley Park was known.

Two years ago commemorative badges were awarded by the government to surviving staff.

The memorial was designed and sculpted by the artist Charles Gurrey.