Invesco Real Estate is buying 20 Manchester Square, W1, from Standard Life Investments for around £70m.
Invesco is understood to have gone under offer to buy the property for a price reflecting a yield of 4.4%.
West End sources said M&G Real Estate, UPP and a Korean investor had all also looked closely at the office development near Marylebone High Street.
The freehold of the 70,000 sq ft four-storey office block is owned by Portman Estate. Standard Life bought the 135-year leasehold from Close Brothers at the end of 2005 for around £60m.
It is let to Boston Consulting Group for a further nine years at a rent of £58 per sq ft and was developed in 2001.
Jones Lang LaSalle is acting for Standard Life.
nick.whitten@estatesgazette.com
