The present invention relates to a process for producing ethylene copolymers in a multistage process comprising at least one slurry phase polymerization stage and at least one gas phase polymerization stage in the presence of Ziegler Natta catalyst comprising a solid catalyst component, a cocatalyst of a compound of group 13 metal and an external additive selected from alkoxysilanes of formula (1) R1nSi(OR2)4-n, (I) where n is an integer 0 to 3, each R1 are equal or different and are selected among H, halogen, alkyl groups of 1 to 6 10 carbon atoms optionally substituted with one or more halogen atoms, alkenyl groups of 2 to 6 carbon atoms optionally substituted with one or more halogen atoms, and aryl groups of 6 to 12 carbon atoms optionally substituted with one or more halogen atoms, or the R1 groups can form with the Si atom they are linked to a ring of 3 to 8 ring atoms, provided that all R1 are not hydrogen, R2 are equal or different and are selected among alkyl groups of 1 to 6 carbon atoms optionally substituted with one or more halogen atoms, alkenyl groups of 2 to 6 carbon atoms optionally substituted with one or more halogen atoms, and aryl groups of 6 to 12 carbon atoms optionally substituted with one or more halogen atoms, or the OR2 groups can form with the Si atom they are linked to a ring of 3 to 8 ring atoms, halogen is Br, Cl or F. The invention further relates to the catalysts and use thereof in said multistage process r for producing ethylene copolymers having melt flow rate ratio FRR21/5 at least 40 and/or polydispersity index PDI of at least 27.