The invention is directed to a process for the production of high density polyethylene by polymerisation of ethylene in the presence of a supported chromium oxide based catalyst and an activator characterised in that the activator comprises the reaction mixture of a boron compound and/or an alkyl aluminium compound and a nitrogen containing compound wherein the boron compound is a (C1-C10) alkyl boron compound or a (C5-C20) aromatic boron compound, wherein the alkyl aluminum compound is an organo aluminium compound having the formula AIR3 in which R is a hydrocarbon radical containing 1-10 carbon atom and wherein the nitrogen containing compound comprises —NH2, —NHR, —NR2, wherein R may be an alkyl or a substituted alkyl having from 1 to 40 carbon atoms.