Royal London is understood to be buying 25 Soho Square, W1, from Aviva Investors for north of its circa £73m asking price.
The property provides more than 42,000 sq ft of office space to tenants including software firm Palantir Technologies, which counts the US Army, United Airlines and Air Asia as clients, Leisure Pass Group and consultant Bow & Arrow.
Aviva acquired the property in 2010 for £40.9m.