Blackstone has exchanged contracts to buy Sanctuary Buildings in Victoria, SW1, from Tishman Speyer.
The property, which was first offered for sale last year, was originally on the market for £184m, reflecting a 5.5% yield.
However, the 227,000 sq ft block now has just three years left on its lease to the Department for Education.
With the DoE already moving staff to the Old Admiralty Buildings, SW1, Blackstone is expected to pursue a major refurbishment.
The sale is a welcome result for Tishman after an earlier deal to sell the building to Henley Investments collapsed this summer.
Tishman held the building in its European core fund and the proceeds of the sale are expected to be recycled into prime office buildings close to Crossrail stations.
The fund has already bought 33 Holborn, EC1 and The Point, W2, this year.
CBRE and GM Real Estate advised Tishman Speyer.
