Will Bennett traces the colourful history of the revolutionary machine that helped shorten the war

Colossus has returned after half a century. Its whirring tapes can read 5,000 characters a second but now the entire capacity of this grandparent of today's computers would fit on to a single modern microchip.

For the past two and a half years, Tony Sale, a former MI5 agent, has been engaged in a labour of love rebuilding the world's first electronic computer, which played a crucial role in shortening the Second World War. Next week his achievement will be recognised when the Duke of Kent switches on the rebuilt Colossus at the museum in Bletchley Park, Buckinghamshire, home of British code-breaking operations during the war.

The ceremony will also mark the reversal of a very British denial of a great achievement: after the war in Europe, Winston Churchill ordered that the Colossus computers be scrapped to "parts no larger than a man's hand".

It was built to crack the German high command's Lorenz code, which was more complex than the Enigma system, which had been penetrated early in the war. Colossus did not decipher the code but identified the wheel settings used by German operators, enabling Bletchley Park experts to work out what Hitler and his generals were discussing. Its greatest triumph was to show they had been fooled by Allied deception plans before D-Day, enabling the landings to go ahead.

Despite American claims that their ENIAC system, which was not running until 1946, was the first computer, British experts have always insisted Colossus was the dawn of a technological revolution that has changed the world.

Mr Sale said: "I wanted to have Colossus up and running this year because the Americans are claiming that ENIAC, a numeric calculator, was first and that this is its 50th anniversary. But Colossus was operating two years before that."

No detailed records of Colossus's construction were retained by the Government: the last drawings were burnt in 1960. Mr Sale, who worked as a computer engineer after leaving MI5, relied on photos, the memories of Bletchley Park experts and inspired guesswork. Many parts used to rebuild the 16ft wide, 7ft 6in high machine, which weighs a ton, are standard equipment used in telephone exchanges during and after the war.

The Government was as uninterested in financing Colossus's rebuilding as it had been in preserving it; a third of the pounds 20,000 cost come from Mr Sale's pocket, the rest from private donations.

He said: "I could not get get any official backing ... I knew I had to do it now, or all the people who had worked on it during the war would soon be dead."