MILAN, Jan 5 (Reuters) - Italian billionaire Leonardo Del Vecchio has further increased his stake in Mediobanca to 11.92%, a regulatory filing showed.

A filing with market regulator Consob showed Del Vecchio's holding company Delfin had bought new shares in Italy's top merchant bank equivalent to 0.92% of Mediobanca's capital in a string of transactions between Dec. 15 and Dec. 30.

The 85-year-old founder of Luxottica in 2018 surprised Italy's financial elites by emerging as the biggest investor in Mediobanca just as a core of historic shareholders unravelled.

Del Vecchio has gained a green light from the European Central Bank to raise his stake to just below 20%. (Reporting by Valentina Za, editing by Francesca Landini)