Cinven will take a 77 per cent share in the provider after acquiring the shares from Phoenix Equity Partners for £123.2m.
The remainder of the shares are owned by management and staff.
The private equity investor plans to inject significant capital into the company, focusing on Partnership’s staff, brand, infrastructure and distribution channels.
Partnership executive chairman Ian Owen says: “Cinven has the investment firepower to enable us now to go much further and much faster.
“They share our vision for the business in building on our market leading mortality data, expertise and reputation to become the undisputed leader in underwritten annuities, whether enhanced or impaired, while continuing to build on our position in the care market.”
Cinven partner Caspar Berendsen says: “This segment of the financial services market is set to grow rapidly driven by some very powerful secular trends such as the switch to defined-contribution pension schemes, changing demographics and encouraging regulatory developments.”