The invention relates to a catalyst system comprising I. a solid reaction product obtained by reaction of: (a) a hydrocarbon solution comprising (1) an organic oxygen containing magnesium compound (2) an organic oxygen containing titanium compound and (3) at least one compound containing zirconium and/or hafnium (b) a mixture comprising a metal compound having the formula MeRnX3-n wherein X is a halogenide, Me is a metal of Group III of Mendeleev's Periodic System of Chemical Elements, R is a hydrocarbon radical containing 1 - 10 carbon atoms and 0 mSiCI4-m wherein 0 < m <= 2 and R is a hydrocarbon radical containing 1 - 10 carbon atoms wherein the molar ratio of metal from (b): titanium from (a) is lower than 1:1 and II. an organo aluminum compound having the formula AIR3 in which R is a hydrocarbon radical containing 1 - 10 carbon atoms. The catalyst is applied during the polymerisation of ethylene, preferably ultra high molecular weight polyethylene.