World War II veterans are preparing to show the public how they cracked the Nazi Enigma codes for the first time since VE Day in 1945.
Bletchley Park's employees worked on a "need to know" basis
A team of 60 enthusiasts has spent 10 years building a working replica of the code-breaking machines that were used to decipher thousands of Nazi messages.
The machines were all dismantled in 1945 and no design drawings survived.
Later this month the replica will be shown at Bletchley Park, in Bucks, where the original codes were cracked.
John Harper, who led the project, said it took so long because there were "so many parts to it".
"The whole thing was to build it as authentically as possible - so we were very lucky that GCHQ provided us with quite a few drawings of individual parts.
"The difficulty was that we couldn't get any assembly drawings so we had to re-create those."
About 10,000 people worked at Bletchley Park at the height of the war - mostly from the Women's Royal Naval Service.
One former employee was 82-year-old Jean Valentine, who described how the original machines "worked beautifully" and sounded like "lots of knitting machines".
She said all of the employees at the code-breaking station worked on a "need to know basis".
"I knew what I was doing but I didn't know what anyone else was doing."
At the time, Prime Minister Winston Churchill commended the women's discretion, reportedly praising them as "the geese that laid the golden eggs but never cackled".
And it was Churchill who, after the war had finished, ordered the machines to be destroyed to keep them out of the wrong hands.
The code-breaking machine - known as the Turing Bombe - was the brainchild of mathematicians Alan Turing and Gordon Welchman.
The Nazi Enigma codes had baffled British and Polish cryptographers because there were so many millions of permutations.
The re-creation took ten years to make from scratch
At the start of the war the code-breakers used Polish machines but later Turing and Welchman redesigned them to enable more than 3,000 enemy messages to be decoded every day.
It is thought the intercepted messages helped the Allies to defeat the Nazis in several crucial battles - so shortening the war by as much as two years.
Simon Greenish, director of Bletchley Park Trust, said the war-time facility was "one of the 20th Century's great stories".
"What was done at Bletchley has affected all our lives in one way or another because World War II would not have ended when it did if it wasn't for Bletchley," he said.
The reconstruction project will be open to the public from July 2007 - although people will be able to see the machine in action at the Churchill and Enigma Reunion weekend on 23 and 24 September.