Underscoring the challenges awaiting Greco after he engineered a rapid turnaround at Generali, Zurich issued a profit warning last week for its general insurance business, its second in four months.

"Like many global players, the company has faced market challenges in recent times but I know that Zurich's strong global franchise, the breadth of talent and the powerful brand provide all of the ingredients for our future success," Greco said in a statement announcing his appointment as CEO at Zurich.

A source had said Greco's decision to leave Generali was due to personal reasons and not to differences with shareholders of the Italian insurer. Regardless, Bernstein analyst Thomas Seidl said his departure was bad news for Generali.

"The market liked Greco and that helped mitigate the downside for the shares. Now there's room for the stock to fall closer to its 13 euro ($14) fair value," Seidl said.

Generali shares closed down 3.15 percent at 14.15 euros on Tuesday, while Zurich shares rose 0.6 percent.

Speculation Greco might move to Zurich has been bubbling since mid-December. He ran the Swiss company's main general insurance business before joining Generali in August 2012 and will return as chief executive on May 1.

Greco's return to Zurich had been reported earlier by Reuters.

TURNAROUND ARCHITECT

Zurich had looked at external candidates for the CEO spot to replace Martin Senn, who quit on Dec. 1 following a failed takeover bid for Britain's RSA. Chairman Tom de Swaan had held the role on an interim basis.

Greco, who started his career at consultancy firm McKinsey, took over at Generali at the height of the euro zone debt crisis after a boardroom coup ousted his predecessor.

The 56-year-old executive, a competitive road cyclist in his free time, is widely seen as the architect of a rapid turnaround that helped almost double the insurer's share price.

Under his stewardship, Generali sold assets worth 4 billion euros, cut costs and bolstered capital ahead of new, tougher European solvency rules.

Generali, 13.5 percent owned by influential investment bank Mediobanca, will have to find a successor to steer it through solvency capital requirements which kicked in this year.

"Generali's capital situation remains unclear and weak relative to peers. We have always argued that Mario Greco should have raised capital right at the start," Seidl said.

A source said the process to find a successor to Greco had started with the insurer looking inside and outside the group.

Generali said its board would meet soon over the matter.

(Additional reporting and writing by Stephen Jewkes, Joshua Franklin and Danilo Masoni; editing by Katharine Houreld and David Clarke)

By Paola Arosio and Joshua Franklin

Stocks treated in this article : Assicurazioni Generali SpA, Zurich Insurance Group Ltd