The present invention relates to a catalyst composition for homopolymerization or copolymerization of olefins comprising: (a) a solid catalyst precursor comprising a transition metal compound of group IVB, VB, or VIB of the periodic table, a magnesium compound and aliphatic polyketone particles; and (b) a cocatalyst comprising an aluminum alkyl, an aluminoxane or mixtures thereof; and a process for homopolymerisation or copolymerisation of olefins.