A multistage process comprising at least one slurry phase polymerization stage and at least one gas phase polymerization stage for producing ethylene copolymers comprising the steps of introducing ethylene, optionally hydrogen and optionally alpha-olefin comonomer having from 4 to 10 carbon atoms into a polymerisation stage A1 in the presence of a solid catalyst Ziegler-Natta component and a cocatalyst, maintaining said polymerisation stage A1 in such conditions as to produce an ethylene homo- or copolymer product P-A1, (a2-i) feeding ethylene, the polymerisation product P-A1 and optionally alpha-olefin comonomer having from 4 to 10 carbon atoms to a polymerisation stage A2, maintaining said polymerisation stage A2 in such conditions as to produce a (co)polymer mixture P-M1 comprising the ethylene (co)polymer P-A1 and a lower molecular weight ethylene (co)polymer P-A2, (c) feeding the (co)polymer mixture P-M1, additional ethylene and an alpha-olefin comonomer having from 4 to 10 carbon atoms, an external additive, optionally hydrogen and optionally additional cocatalyst to a polymerisation stage B (d) maintaining said polymerisation stage B in such conditions as to produce a higher molecular weight polymerisation product P-B, (e) recovering the polymerisation product P-B from the polymerisation stage B, wherein the external additive has formula R 1 n Si(OR 2) 4-n, where n is an integer 0 to 3, each R 1 are equal or different and are selected among H, halogen, 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 R 1 groups can form with the Si atom they are linked to a ring of 3 to 8 ring atoms, provided that all R 1 are not hydrogen, R 2 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 OR 2 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, and wherein the polymerization stage B is a gas phase polymerization stage; wherein the external additive is fed only to the polymerization stage B; and wherein the polymerisation product P-B is an ethylene copolymer having a polydispersity index PDI of at least 27. introducing ethylene, optionally hydrogen and optionally alpha-olefin comonomer having from 4 to 10 carbon atoms into a polymerisation stage A1 in the presence of a solid catalyst Ziegler-Natta component and a cocatalyst, maintaining said polymerisation stage A1 in such conditions as to produce an ethylene homo- or copolymer product P-A1, (a2-i) feeding ethylene, the polymerisation product P-A1 and optionally alpha-olefin comonomer having from 4 to 10 carbon atoms to a polymerisation stage A2, maintaining said polymerisation stage A2 in such conditions as to produce a (co)polymer mixture P-M1 comprising the ethylene (co)polymer P-A1 and a lower molecular weight ethylene (co)polymer P-A2, (c) feeding the (co)polymer mixture P-M1, additional ethylene and an alpha-olefin comonomer having from 4 to 10 carbon atoms, an external additive, optionally hydrogen and optionally additional cocatalyst to a polymerisation stage B (d) maintaining said polymerisation stage B in such conditions as to produce a higher molecular weight polymerisation product P-B, (e) recovering the polymerisation product P-B from the polymerisation stage B, each R 1 are equal or different and are selected among H, halogen, 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 R 1 groups can form with the Si atom they are linked to a ring of 3 to 8 ring atoms, provided that all R 1 are not hydrogen, R 2 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 OR 2 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, and wherein the polymerization stage B is a gas phase polymerization stage; wherein the external additive is fed only to the polymerization stage B; and wherein the polymerisation product P-B is an ethylene copolymer having a polydispersity index PDI of at least 27.