The present invention relates to a process for the radical polymerization or copolymerization of ethylene under high pressure in the presence of at least one polymerization initiating-controlling compound capable of providing, by decomposition under the polymerization or copolymerization conditions: at least one initiating free radical (Z) which carries at least one site for initiating the (co)polymerization; and at least one stable free radical (SFR) which carries at least one site exhibiting the stable radical state and which is stable under the polymerization conditions, with, in total, as many initiating sites as sites exhibiting the stable radical state.