Colossus was rebuilt over 14 years at Bletchley Park
Bletchley Park, the hub of Allies' code-breaking during WWII, is in a financial crisis, managers have said.
The Milton Keynes centre built the first modern computers, known as Colossus machines, to crack German codes in the 1940s.
But the Bletchley Park Trust said the centre had fallen into decay and with no on-going public funding could not afford repairs costing more than £1m.
The charity said an appeal has been made to the Heritage Lottery Fund.
It said the main Victorian mansion requires in the region of £1m for repairs to the roof and some of the code-breaking huts are in a desperate state of decay.
Known as the National Codes Centre, Bletchley Park re-built one of its Colossus machines, which was completed last year and put it to the test in a code-breaking challenge.
It took three hours longer than a modern computer to crack a message enciphered with a Lorenz S42 machine as used by the German high command in WWII.
But Colossus, which was destroyed on the order of Sir Winston Churchill after the war, is widely recognised as the first modern programmable computer.
Simon Greenish, director of Bletchley Park Trust, said: "The site is unique and one of the most important remaining from World War II.
"We have exciting plans to develop the park and save it for future generations. The more visitors we have helps us to realise these plans."