Del Vecchio, the 85-year-old founder of spectacles maker Luxottica, last year emerged as the largest shareholder in Mediobanca, sending shockwaves through Italy's financial world.
Mediobanca, which in turn is the single biggest investor in insurer Generali, had not had a single non-banking shareholder owning more than 10% since it was founded in 1946.
A regulatory filing on Wednesday showed Del Vecchio held 10.162% of Mediobanca at Oct. 5, up from 9.89% previously, through his holding company Delfin.
The billionaire won European Central Bank approval to raise his holding to up to 19.9% of Mediobanca at the end of August. Sources had told Reuters at the time that Del Vecchio would increase his stake only slightly in the short term.
Delfin, which under Italian rules was obligated to disclose its future plans after crossing the 10% threshold, said its Mediobanca stake was a long-term financial investment aiming to promote the Milanese bank's growth and stability.
Delfin said it would assess whether to raise its stake over time "based mainly on the return of the investment, market conditions and opportunities to buy shares".
"Delfin does not intend to gain control [of Mediobanca] or anyway exercise a dominant influence on its management," it said.
Del Vecchio's move comes as Mediobanca prepares for a shareholder vote to keep current boss Alberto Nagel in place as it appoints a new board on Oct. 28.
Delfin said it would not propose any board changes and would not submit its own list of board nominees.
Mediobanca's outgoing board has presented a list of directors for the AGM, confirming Nagel and Chairman Renato Pagliaro as directors. Mediobanca's core shareholders, holding in aggregate 12.6% of the bank, are expected to back the list.
Two other lists were submitted by minority investors.
(Reporting by Claudia Cristoferi and Gianluca Semeraro; Editing by Valentina Za and David Goodman)