Bramson has so far failed in attempts to get the lender to scale back its investment banking activities, which he says have weighed on shareholder returns and run the risk of the bank needing to raise fresh capital.

While Barclays has said it remains committed to its strategy, Chief Executive Jes Staley last month ousted the head of its investment banking division and instead took direct control of the unit, a move referenced in Sherborne's latest letter.

"In view of Barclays' most recent announcements of sudden management realignments and departures, we believe that Mr. Bramson's experience and temperament would be a strongly stabilizing influence on the board," the letter said.

Barclays is due to hold its annual general meeting of shareholders on May 2. Sherborne has a stake of around 5.5 percent in the bank.

(Reporting by Simon Jessop; Editing by Keith Weir)

Stocks treated in this article : Barclays PLC, Sherborne Investors (Guernsey) B Ltd