MOSCOW, March 20 (Reuters) - Russian online bank Tinkoff may take on Rosbank's retail client base as part of proposed integration that could strengthen billionaire Vladimir Potanin's foothold in Russia's banking sector.

Rosbank board chairman Nikolai Sidorov on Wednesday said the most obvious benefit from the two banks integrating would come from combing their retail businesses on Tinkoff's platform.

Tinkoff Bank owner TCS Holding, controlled by Potanin's Interros group, is seeking to integrate Rosbank into the holding to increase its share of Russia's financial market.

Potanin's Interros, one of the most powerful private investment companies in Russia, bought a controlling stake in Rosbank from France's Societe Generale in 2022, followed by a 35% stake in TCS from businessman Oleg Tinkov.

Potanin said the banking group's future was still being discussed but that both lenders would retain their licences, brands and teams, with Rosbank continuing to focus on corporate and private clients and Tinkoff looking to expand its 40-million-strong retail client base.

"This is the very situation in which one plus one gives more than two," Potanin was quoted as saying by Interros.

TCS Holding's board of directors has decided to initiate a buyback of up to 19 million of its own shares on the market, representing about 10% of the company's share capital, it said last week.

On Wednesday, the holding's board proposed issuing an additional 130 million shares at 3,423.62 roubles per piece, with a shareholder meeting set for May 8.

"The banks' integration will take place on the basis of fair market valuation, with Tinkoff being valued at an adequate premium to the market," Potanin was quoted as saying. (Reporting by Elena Fabrichnaya and Alexander Marrow; editing by David Evans)