Mediobanca shareholders vote on Oct. 28 to pick new directors and the outgoing board is expected to propose another three-year mandate for Chief Executive Alberto Nagel and Chairman Renato Pagliaro.

Delfin's decision to file a separate list of candidates increases uncertainty over Mediobanca's governance in the short term and could lead to a less cohesive board down the road, analysts said.

Nagel has led Mediobanca since 2008, while Pagliaro has chaired its board since 2010.

Delfin, which in recent years has acquired nearly 20% of Mediobanca while committing with banking supervisors not to interfere with strategy, would like to see a fresh face in the form of a new chairman, the sources said.

Delfin has until Oct. 3 to file its list and has to decide how many candidates to put forward. It is not expected to put up a direct challenger to Pagliaro.

"The main risk is that of a fragmented board ... which complicates the running of the bank," broker Equita said.

Delfin's move, which could be defused by a last minute compromise that the sources however said was unlikely, comes after the failure of previous attempts to reach an accord with the existing board over a single slate of nominees.

Mediobanca's nomination committee meets later on Monday to finalise the proposal to keep Nagel and Pagliaro at the helm, which the board is expected to approve on Wednesday.

Mediobanca has stuck to its governance rules which prevents the board from choosing a chair in accordance with a shareholder.

Under Nagel, Mediobanca has moved away from its historic role of powerhouse of Italian capitalism with ties to all the top companies through a web of cross shareholdings.

With Mediobanca for 17 years prior to taking charge, CEO Nagel has diversified the business to range from investment banking to consumer finance and wealth management.

He streamlined the financial holding portfolio to a 13.13% stake in insurer Generali which remains a key contributor to earnings -- a reliance which late Delfin founder Del Vecchio had in the past said it was excessive.

(Reporting by Claudia Cristoferi; writing by Valentina Za; Editing by Keith Weir)

By Claudia Cristoferi