MILAN, July 20 (Reuters) - Italian businessman Francesco Gaetano Caltagirone has raised his holding in Mediobanca fivefold, a regulatory filing showed on Tuesday, as speculation mounts over the fate of the illustrious Milanese merchant bank.

A filing with market regulator Consob showed Caltagirone, 78, held 2.88% of Mediobanca as of July 12, while owning another 2.175% through put options, for an overall potential stake of 5.055%.

In March, a Consob filing had revealed Caltagirone, a construction and newspaper magnate, had bought 1% of Mediobanca.

Mediobanca is Italy's leading investment bank. Once a powerhouse that called the shots in the country's corporate world, Mediobanca is also the largest investor in top insurer Assicurazioni Generali.

Caltagirone is the second-largest shareholder in Generali and his twin investments in both Mediobanca and Generali mirror those of fellow billionaire, octogenarian Leonardo Del Vecchio.

Their stake-building has raised questions about the future of both Mediobanca and Generali at a time when consolidation is sweeping through Italy's banking and insurance sectors.

The two firms are among Italy's most prized financial assets. They have both been under the spotlight since Del Vecchio, founder of Ray-Ban owner Luxottica, in 2019 emerged as the biggest investor in Mediobanca while also being Generali's third-largest shareholder behind Caltagirone.

Del Vecchio, 86, earlier this month raised his Mediobanca stake to 18.9% (Reporting by Valentina Za and Gianluca Semeraro; editing by Richard Pullin)