GCHQ releases the secret details of how Bletchley Park built the first computer

The machine was affectionately nicknamed the Heath Robinson and even some of the codebreakers at Bletchley Park were dubious about its usefulness. But the work they embarked on turned this rudimentary device into Colossus, the world's first computer, which allowed the Allies to "read Hitler's mind", shortening the Second World War, it is claimed, by two years.

The machine was affectionately nicknamed the Heath Robinson and even some of the codebreakers at Bletchley Park were dubious about its usefulness. But the work they embarked on turned this rudimentary device into Colossus, the world's first computer, which allowed the Allies to "read Hitler's mind", shortening the Second World War, it is claimed, by two years.

Fifty-seven years after the prototype was first demonstrated, hitherto classified documents telling the full story of the creation of Colossus were released to the Public Record Office yesterday by GCHQ, the government centre that spies on foreign communications.

The 500-page report, The History of Newmanry, was named after Max Newman, the mathematician who headed the special unit at Bletchley Park responsible for deciphering enemy intelligence. It charts how the creaking problems of the Heath Robinson machine were overcome to read the German code systems, Enigma and Lorenz.

When it was fully functional, Colossus weighed over a ton and occupied an entire room. But, say scientists, it was astonishingly ahead of its time and opened the door to the age of information technology.

The documents show how, as the Germans and Japanese swept through their respective theatres of war, the Bletchley Park codebreakers were working away. Heath Robinson had been the brainchild of Alan Turing, the Cambridge academic who was to play a central role in breaking Enigma.

But the equipment had huge difficulties in unravelling the Lorenz system, with its code, Fish, the enciphered teleprinter communications used by Hitler to talk with his generals. Max Newman made headway in tackling Lorenz, but there were still major problems.

Then, in 1943, Turing recalled the work being done by Tommy Flowers with the telephone system for the Post Office at Dollis Hill. He was called in and, after examining Heath Robinson, proposed that the mechanical switching units for the machine should be replaced by electronic valves.

There was, at first, fierce opposition from the codebreakers, who were convinced that the valves would keep breaking down. But Flowers knew otherwise from his Post Office experience and managed to persuade a sceptical Bletchley Park to give his solution a try. Thus, the first programmable electronic computer was conceived.

There were still major hurdles to be overcome. Flowers said the minimum time for producing his version of the machine was a year. But he was bluntly told by Churchill and the general staff - who were reluctant to reveal that they wanted it before the still-secret D-Day operation - that a year would be too late because, by then, Britain would be under Nazi occupation.

While Bletchley Park continued with Heath Robinson, Flowers pursued his own work and within 10 months, often working through the night. he unveiled Colossus to the codebreakers at a demonstration on 8 December 1943.

Colossus contained 1,500 valves, 10 times more than other electronic machines of the day. It was designed torun through millions and millions of possible settings forthe code wheels on the German enciphered teleprinter system, processing 5,000 characters a second. What was even more astonishing for the codebreakers was its accuracy compared to the Heath Robinson.

With the Bletchley Park team now enthusiasts for Colossus, progress continued at a rapid rate. The German Lorenz code was soon being broken semi-automatically, thanks to proposals by Donald Michie, now the emeritus professor of machine intelligence at Edinburgh University, then a 20-year-old undergraduate. Another young man, Harry Fensom, now aged 79, was brought in to help develop Colossus II, which had to be ready in time for D-Day.

The project was successful, Colossus II, which had 2,500 valves, came into operation on 1 June 1944, just in time to reveal that Hitler, against the advice of some of his commanders, had swallowed Allied misinformation about where and when the retaking of Europe would begin. This gave Eisenhower and Montgomery the confidence to tell Roosevelt and Churchill thatD-Day should proceed as planned.

By the end of the war there were 10 Colossus computers operating in Britain, being worked by three shifts around the clock. By then, Italian and Japanese codes, as well as German, had been broken. But with victory came orders, at the insistence of Churchill, that all but two of the Colossus computers should be destroyed and their existence kept a secret. There was widespread fear that leaking of the details would remove Britain's advantage in intelligence gathering.

The two surviving machines were installed at the new GCHQ at Cheltenham, Gloucestershire, where one was still operating in the early 1960s against the new enemy, the Soviet Union.

The Russians were aware of its use, but KGB documents reveal almost fond references to the computer, acknowledging its role in defeating the Nazis, the common enemy of the past.

Partly as a result of this secrecy, however, the Americans were able to claim, wrongly say the experts, that the first electronic digital computer was the US built Eniac, which became operational in 1946.

Although some details about the computer had begun to become public by the Seventies, the men and women who worked on Colossus had to remain silent about much of their work. Many of them are no longer alive to enjoy the belated public acknowledgement of their extraordinary triumph.