The move sees previous investor Cabot Square Capital sell its majority stake after six years. The deal is subject to regulatory approval.
Phoenix Equity Partners is investing along with the Key Retirement management team of chief executive Colin Taylor, managing director Paul Wilson and group directors Dean Mirfin and Richard Overson.
It said the investment by Phoenix would help accerlerate growth and the company hopes to launch new products and possible acquisitions. Recent launches have included the launch of a home buying and selling service called Key Move Property Services.
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Colin Taylor, chief executive of Key Retirement, said: "We are excited about partnering with Phoenix, a firm with considerable experience of investing successfully in the retirement segment of financial service, to continue to build KRS into a trusted brand for the over-55s."
James Clark, partner at Cabot Square Capital, said: "KRS has been a fantastic success story over the last six years, expanding organically and through acquisition to provide multiple offerings for people in or approaching retirement.
"KRS is well placed to capitalise on this momentum and we wish them all the best for the next stage of their growth."

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