The company is understood to have placed 25 North Colonnade under offer for around £15m below the initial price set by its owners Evans Randall when it was placed on the market in March.
The deal comes just months after the City watchdog confirmed plans to move its 3,000 staff from Canary Wharf to a new £2bn development next to the Olympic Park known as the International Quarter.
The move was a major coup for the developers of the 22 acre site – Australian giant Lend Lease and the taxpayer-owned London and Continental Railways (LCR) – who aim to turn the Stratford area into a new office district.
The FCA is set to move out of 25 North Colonnade in 2018 when it lease expires after almost 20 years in the building.
Evans Randall’s sale to Blackstone will put and end to months of uncertainty for the landlord after failed to repay a £205m loan when it became due in October last year.
Both parties declined to comment.