MILAN (Reuters)—Opponents of Assicurazioni Generali S.p.A. Chief Executive Giovanni Perissinotto are on the brink of unseating him in what they see as a key step in reviving the appeal of Italy's top insurer after months of underperformance.

Top investor Mediobanca S.p.A., which is leading a coup against the long-serving Mr. Perissinotto, appears to have mustered enough support to oust him at a board meeting on Saturday, sources close to the board told Reuters on Friday.

The chief executive has survived bitter boardroom battles leading to the ousting of former chairmen Antoine Bernheim in 2010 and Cesare Geronzi in 2011 but now seems doomed by concerns shared by a majority of board members and large investors.

Zurich Insurance Co. Ltd. executive Mario Greco, who used to lead Italian insurer RAS and asset manager Eurizon, could be placed at the helm of Europe's No.3 insurer.

"There is a broad agreement among board directors and investors to replace Perissinotto," one of the sources told Reuters on Friday, a view shared by three others.

Generali investor Leonardo Del Vecchio said Friday that Mr. Perissinotto was incapable of running the group and backed management changes.

However, one board member said the situation was still fluid.

Mr. Perissinotto would be the second leading European insurance boss to be forced out by investors in a month, following the departure of Aviva P.L.C. CEO Andrew Moss on May 8.

Both have been blamed for persistent share-price weakness driven as much by their companies' exposure to the heavily indebted countries at the center of the eurozone crisis as any management shortcomings, said one London-based analyst.

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Mr. Perissinotto, 58, defended in an angry letter to board directors his performance during his 10-year tenure, saying Generali has been of late mainly suffering from the eurozone crisis and adding that his ouster would rock Italy's biggest financial group at a time of great market volatility.

He said Mediobanca's main reason for trying to get rid of him was his own independence in the face of the investment bank, which is the insurer's main investor with a 13.5% stake.

But shares rallied on the news, suggesting investors would welcome a new face at the help of the Trieste, Italy-based insurer.

The clash between Mediobanca and Generali's CEO was the latest example of how its interests as a bank can sometimes conflict with its role as a Generali shareholder.

It also underlines its continued influence at the insurer despite a recently passed Italian law seeking to untangle the web of Italian finance's cross shareholdings.

In the letter, Mr. Perissinotto harshly criticized Mediobanca's attempt to merge stricken insurer Fondiaria-SAI S.p.A., which owes over a billion euros to Mediobanca, with smaller peer Unipol Gruppo Finanziario S.p.A., a contested deal now under regulatory scrutiny.

The CEO said he was being sacrificed because he had not tried to derail a rival rescue deal for Fondiaria-SAI led by private equity funds Sator Capital Ltd., led by banker Matteo Arpe, and Palladio Finanziaria S.p.A., this latter seen close to the manager.

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"It is obvious that the belief that I have somehow helped, or rather, not used my influence to block one player from joining a transaction that threaten Mediobanca's vital interests, is at the basis of the no confidence vote," he wrote.

Generali is a large source of income for Mediobanca, which through this holding is able to wield influence across Italy's entire financial landscape.

The progress of investments in Russia and eastern Europe have increasingly irked Mediobanca and other investors.

Shares in Generali, down 30% in the year to date, rallied 4% in reaction to the news on Friday against a flat European insurance index, with some traders saying the market would welcome the arrival of Greco.

Germany's Allianz S.E. and AXA S.A. of France, its two main rivals, have fallen 22%.

"The departure of Perissinotto and the arrival of Greco, a man well-respected by the market, would make the stock more appealing," a Milan-based trader told Reuters.

Some analysts were skeptical of the ability of a new manager to turn the insurer around amid euro zone woes.

Credit Suisse analysts defended Mr. Perissinotto's work: "It is not obvious to us that a new CEO could do anything to change the fact that Generali by its nature as the largest insurer in Italy has a disproportionate exposure to Italian government debt."

Banks and insurers based in weaker euro zone members have been struggling due to the deepening eurozone crisis. Generali had €46 billion ($57.58 billion) of Italian government bonds in late 2011.