Late last year, CGNU agreed to sell-off Marlborough Underwriting, its mainly marine managing agency for less than £10m - just three months after increasing its stake in the firm from 51% to 100%.
Marlborough was bought by the Berkshire Hathaway Group, controlled by US billionaire Warren Buffett.
With its ownership of Marlborough, Berkshire Hathaway will be a sizeable force at Lloyd's. It already controls managing agency DP Mann, which has a capacity of £257m. With Marlborough, it will control 5% of capacity at Lloyd's.
Bob Scott, CGNU chief executive, says the deal will protect the insurer from "further exposure" related to its London market risks, including Lloyd's.
The sale of Marlborough ends CGNU's involvement in the London insurance market and follows its exit from the corporate risks market in August.