If the council decides not to extend the boundary of the existing conservation area, the buildings where more than 80,000 German codes a month were cracked in the build-up to D-Day in June 1944 face imminent demolition to make room for 450 houses.
The building which housed Colossus, the world's first electronic computer, developed during 1943, has already been demolished.
The site is unique, not just for the history of cryptology (code deciphering) but also for computing, radar and air traffic control.
Leading the campaign to save the site is the Bletchley Park Trust, a body supported by voluntary donations. The trust hopes to raise pounds 7m to purchase the site and its historic manor house and open it as a 'campus museum' for scholars and the general public. The trust says ample land, and planning permission, already exists for 20,500 new homes in Milton Keynes.
The land is under joint British Telecom and Government ownership. In February 1992, after protests from the trust, the council placed a conservation order on the southern part of the site, containing earlier blocks.
Tonight's decision concerns the northern part of the site, currently unprotected. It contains blocks D, F, G and H, which were crucial to the D-Day effort. Blocks D and G are bomb-proof buildings where military planners met in January 1944 to plan intelligence support for the Normandy landings.
British Telecom has made a separate application to demolish all the buildings in the southern part of the site as well. The only piece permanently preserved is the manor house itself.
At the peak of its activity Bletchley Park employed 12,000 people.
Enemy signals were intercepted all over the globe, and also at the Secret Intelligence Services station four miles away, and fed to Bletchley by teleprinter link.
For a brief period around the Normandy invasion Bletchley also sprouted aerials to intercept communications.
The site is not formally open to the public but Ted Enever, chief executive of the trust, conducts guided tours.
He said: 'We have appealed to Government to donate the Bletchley Park site to the nation so that we can preserve and commemorate the work of the code- breakers who helped shorten the war through brains rather than bullets.'