Completed in 1962, the massive office complex - birthplace of the laser and the mobile phone - could be torn down, at least in part, if a controversial property deal goes through later this summer.
The proposed deal has provoked outrage from a spectrum of groups keen to see the 'cathedral-like' campus spared from the bulldozers.
However, PREI is claiming that the current building stock is not suitable for modern-day uses. A spokesman said: 'While it's beautiful from an architectural standpoint, from a pragmatic stand it doesn't hold up.
'The building is, for all intents and purposes, functionally obsolete in today's commercial market.
'The building was... constructed in a manner that would make modernisation extremely costly, if not entirely implausible.'
Even so, the developer has maintained that, in a nod to the past, phase one of Saarinen's original design will be preserved.
The sale is expected to go through in August 2007.