The present 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 which is modified with an organic compound comprising oxygen and a nitrogen atom which is selected from the group consisting of saturated heterocyclic organic compounds with a five or six membered ring, amino esters and amino alcohols. Such organic compounds allow manufacturing of HDPE with increases molecular weight distribution and increased molecular weight.