The open letter said the share price of St James's Place (SJP) failed to reflect a doubling of client assets over the last five years. It cited "a bloated organisational structure, excessive hiring, excessive pay and mounting losses in Asia with little prospect of a recovery."

Despite funds under management growing 18% a year since 2015, the share price has fallen 7% and total shareholder returns were 2% annualised, it added.

In response, SJP said it "proactively engages with shareholders with regards to group strategy and structure and looks forward to commencing a dialogue with PrimeStone in regard to the views outlined in its letter."

Shares in SJP, which will give its latest trading update on Tuesday, were flat on Monday in line with the broader market.

London-based PrimeStone, founded by former leading private equity dealmakers at The Carlyle Group, said it now owned 1.2% of St James's Place stock and was going public after failing to get the board to address its concerns "head on".

"SJP has delivered tremendous value for clients, advisers, employees and management ... but not so much for shareholders over the last five years," fund partners Benoît Colas and Damian Hahnloser said in the letter.

"It is time for the company to address its high cost base and change its culture in order to deliver its full value-creation potential to long-neglected owners."

The letter called on SJP management to conduct an in-depth cost review and set "ambitious" targets to bring its cost base to the same level as best-in-class peers. It called for the plan to be presented and discussed with investors, and for the company to regularly report on its progress.

It also called for better financial communications with clearer reporting on revenues, costs, margins and important Key Performance Indicators.

(Reporting by Simon Jessop; Additional reporting by Tanishaa Nadkar; Editing by Rachel Armstrong and Mike Harrison)

By Simon Jessop