The bid is almost £50 million more than the original asking price for the 1950 building in Savile Row, and far outstrips a number of previous offers.
Although the identity of the bidder is currently unknown, the AJ understands the developer-led team is not the one which includes interior designer and conservation specialist Oliver Laws.
Oliver Laws had come up with proposals to convert the building into an 80-bedroom hotel, and fears are now growing among conservationists and heritage groups that any other plan would see the building bulldozed.
Eric Parry has already tabled proposals to flatten the block and replace it with a new office building.
Other architects who have recently been linked with the site include French star Jean Nouvel.
The latest rumours come only days after campaigners hoping to save Fortress House welcomed news that property developer the Raven Group had made an 'improved' offer to buy the building ( Future of Fortress House hangs in the balance).
Headed up by Anton Bilton, the group had worked on a scheme with American hotelier André Balazs to create a hotel and residential development within the existing building.
Legal & General refused to comment.