To look at, it is a distinctly undistinguished and desolate-looking piece of utilitarian 1940s architecture. There is a failed concrete roof, water seeping in, pigeons nesting and vegetation growing through the cracks in the brickwork.

But in context, the long overlooked and derelict Block C at Bletchley Park, the second world war codebreaking headquarters in Buckinghamshire, may be one of the most significant buildings of the 20th century and on Friday the government gave it Grade II listed status to preserve it for posterity.

It joins most other buildings on the site, including the famous wooden huts in which scientists, intelligence officers and civil servants, led by figures such as Alan Turing, initially worked to break the Nazis' supposedly impenetrable Enigma code. Their success shortened the war by as much as four years, by some accounts.

John Penrose, the heritage minister, said: "The work that took place at Bletchley Park was absolutely key to the allied victory. Block C was vital to the success of Bletchley Park's codebreaking work. But the site also had a huge role in the start of the information age and can be viewed as the birthplace of modern information technology."

Block C's significance lies in the fact that after it was built in 1942, it housed the early high-speed digital processing Hollerith sorting machines, used to sort and analyse data. It is therefore the forerunner of later computer processing developments and, in the words of English Heritage's report: "A uniquely important site, arguably as significant to the information age as Ironbridge is to the industrial revolution."

Although the building was later partitioned into offices and classrooms – it was last used as a training centre by GCHQ in 1987 and has been empty ever since – Block C was originally a 2,800 sq metre (30,000 sq ft) open-plan space, soundproofed with fibrewood ceiling tiles to muffle the sound of its machinery, which worked through the night. Eight wings ran off the central core to give workers more light and provide offices and kitchens.

The plan now is for Block C to be rehabilitated as part of the site's £7.5m renovation project, to become the park's entrance and visitor centre in 2014. The Bletchley Park Trust has received Heritage Lottery funding of £5m but is still raising money to complete the work. Even in its dilapidated state, it still received more than 140,000 visitors last year.

Simon Greenish, the trust's chief executive, said: "These buildings are not architectural monuments of any style at all, but their historical significance is very important. Listing Block C is an important step forward."