Catalyst system comprising an ionic metallocene derived from a transition metal, a catalytic solid containing an element of group IVB, magnesium and a halogen, and an aluminium derivative.Process for preparing this catalyst system, according to which a mixture of a neutral metallocene and of the catalytic solid is prepared, the mixture thus obtained is placed in contact with an aluminium derivative and an ionizing agent is added to it.Process for (co)polymerization of an olefin, according to which a mixture of the neutral metallocene and of the catalytic solid is prepared, the mixture thus obtained is placed in contact with an aluminium derivative and the olefin, and the ionizing agent is added to it.