Solid catalyst with a high thermal stability for the (co)polymerization of α-olefins, comprising titanium, magnesium, at least one metal selected from hafnium and zirconium, aluminum and chlorine, wherein at least 60% of the titanium is in oxidation state +3, and, when examined by means of XPS spectroscopy, has an absorption band characteristic of a binding energy ranging from 455 to 458 eV. Said catalyst, used in combination with a suitable co-catalyst in (co)polymerization processes of α-olefins at a high temperature, shows an improved productivity, a high incorporation of co-monomers in the copolymerization of ethylene and an increased thermal stability with respect to the systems so far in use.