A porous solid catalyst component comprising a magnesium halide, a titanium compound having at least a Ti-halogen bond and at least two electron donor compounds one of which being selected from 1,3-diethers and the other being selected from succinates, characterized by the fact that the molar ratio ID/Ti is from 0.30 to 0.90, where ID is the total molar amount of succinate and 1,3-diether, the molar ratio of the 1,3-diether donor to the succinate donor is higher than, or equal to, 0.60.