Key RCS Mediagroup shareholders meeting seen Sept 21

CERNOBBIO, Italy, Sept 7 (Reuters) - The controlling shareholders of Italian publisher RCS Mediagroup are likely to meet on Sept. 21, one of them said on Friday, amid talk the loss-making company is planning a capital increase.

Italian media have speculated the publisher of the Corriere della Sera newspaper might seek 400 million euros of capital, forcing the shareholder group to either take part or see its stake diluted. The company must disclose its plans by Oct. 16.

RCS is 58-percent-owned by stalwarts of Italy's business establishment including Mediobanca, Fiat's Agnelli family, the Merloni family, tyremaker Pirelli, Intesa Sanpaolo, and Generali.

The group may meet on Sept. 21, said Francesco Merloni, who owns a 2 percent stake.

It is too soon to talk about a capital increase for RCS, said Paolo Merloni, a board member.

The company has seen its shares surge after its new CEO said in August he was working on a turnaround plan. {ID:nL6E8J7CAD]

The increase has also been fuelled by speculation that control of the tightly-held group could splinter under the weight of its losses as some shareholders opt out of a possible capital increase, making the company vulnerable to a takeover.

RCS punches well above its market weight in terms of media clout, since Corriere is perceived to influence voter opinion in the runup to Italy's election next year.

The company's illiquid stock has surged 375 percent in the past month, hitting a 52-week high on Sept. 6. Shares closed down 28 percent on Friday at 1.55 euros.

Separately, the Milan prosecutor's office on Friday received a complaint of alleged insider trading on the stock, said Luigi Giuliano Martino, the lawyer who presented the complaint.

Martino said the complaint was made by a person who wanted to remain anonymous.

The complaint asks that "public opinion be informed of the identities of who bought and sold the stock and whether or not these purchases were concentrated in the hands of a small number of brokers." (Reporting by Gianluca Semeraro, Elisa Anzolin, Illaria Polleschi; Editing by Mark Potter)