The National Museum of Computing at Bletchley Park, Buckinghamshire, UK is marking the 75th anniversary of the Colossus Mark II computer today with new revelations about its role in winning the Second World War. At a gathering of those who operated what was one of the world's first electronic computers, the museum will disclose how intelligence gathered by the code-breaking machine helped the Allied war effort in ways that even the computer veterans were unaware of.

On June 1, 1944, the Colossus Mark II computer went online. It was the second of the 10 Colossus machines that were built by the British to break the German Lorenz ciphers that were used to encrypt top secret messages from the Nazi high command and Adolf Hitler himself. Less well-known than the more famous Enigma code, Lorenz was much more complicated and much harder to break.

The previous electromechanical decrypting machines weren't up to the task, so the Bletchley Park team under Thomas "Tommy" Flowers, a General Post Office (GPO) telephone electrical engineer who'd been developing a digital electronic telephone switching system using radio valves, created the first true electronic computer, Colossus.

Colossus Mark I booted up on February 5, 1944 and was so successful that more of the computers were ordered. The Mark II went into service on June 1. According to the museum, this may have been so that the computer could handle the German code traffic during the build up to the still-secret D-Day invasions by the Allied forces, which were to land on the beaches of Normandy on June 6.

Because of the strict secrecy that surrounded the decoding project, the very existence of Colossus was unknown to almost everyone outside the intelligence services until the 1980s, and even during the war the Wrens (Women's Royal Naval Service) had no idea of the significance of their work tending the computer. As part of today's ceremony, the surviving women of the team and others will learn whole story.

The museum says that the event speakers will discuss the pressures of the D-Day code-breaking effort, as well as the results of their National Archives research on the intelligence results from Colossus for the Allied landings – especially how it confirmed that the Germans had fallen for the Allied disinformation campaign to fool the Axis as to when, where, and how the invasion would come.