A process for preparing a procatalyst suitable for preparing a catalyst composition for olefin polymerization, said process comprising the steps of: i) contacting a compound R 4 z MgX 4 2-z with an alkoxy- or aryloxy-containing silane compound to give a first intermediate reaction product, being a solid Mg(OR 1) x X 1 2-x, wherein: R 4 is the same as R 1 being a linear, branched or cyclic hydrocarbyl group independently selected from alkyl or aryl groups, and one or more combinations thereof; wherein said hydrocarbyl group may be substituted or unsubstituted, may contain one or more heteroatoms and preferably has from 1 to 20 carbon atoms; X 4 and X 1 are each independently selected from the group of consisting of fluoride (F-), chloride (CI-), bromide (Br-) or iodide (I-), preferably chloride; z is in a range of larger than 0 and smaller than 2, being 0 < z < 2; x is in a range of larger than 0 and smaller than 2, being 0 < x < 2; ii) optionally contacting the solid Mg(OR 1) x X 1 2-x obtained in step i) with at least one activating compound selected from the group formed by activating electron donors and metal alkoxide compounds of formula M 1 (OR 2) v-w (OR 3) w or M 2 (OR 2) v-w (R 3) w, to obtain a second intermediate reaction product; wherein: M 1 is a metal selected from the group consisting of Ti, Zr, Hf, Al or Si; M 2 is a metal being Si; v is the valency of M 1 or M 2, w is smaller than v; R 2 and R 3 are each a linear, branched or cyclic hydrocarbyl group independently selected from alkyl, alkenyl, aryl, aralkyl, alkoxycarbonyl or alkylaryl groups, and one or more combinations thereof; wherein said hydrocarbyl group may be substituted or unsubstituted, may contain one or more heteroatoms, and preferably has from 1 to 20 carbon atoms; iii) contacting the first or second intermediate reaction product obtained in step i) or ii) respectively, with a halogen-containing titanium compound and either an activator or an internal electron donor to obtain a third intermediate reaction product; iv) modifying the third intermediate reaction product obtained in step iii) with a modifier having the formula M(p)X p, preferably MX 3, wherein M is a metal selected from the Group 13 metals and transition metals of the IUPAC periodic table of elements, p is the oxidation state of M, and wherein X is a halide, to yield a modified intermediate reaction product; v) contacting said modified intermediate reaction product obtained in step iv) with a halogen-containing titanium compound and in the case that in step iii) an activator was used an internal donor to obtain the procatalyst, wherein preferably step v) is carried out at least two times. i) contacting a compound R 4 z MgX 4 2-z with an alkoxy- or aryloxy-containing silane compound to give a first intermediate reaction product, being a solid Mg(OR 1) x X 1 2-x, wherein: R 4 is the same as R 1 being a linear, branched or cyclic hydrocarbyl group independently selected from alkyl or aryl groups, and one or more combinations thereof; wherein said hydrocarbyl group may be substituted or unsubstituted, may contain one or more heteroatoms and preferably has from 1 to 20 carbon atoms; X 4 and X 1 are each independently selected from the group of consisting of fluoride (F-), chloride (CI-), bromide (Br-) or iodide (I-), preferably chloride; z is in a range of larger than 0 and smaller than 2, being 0 < z < 2; x is in a range of larger than 0 and smaller than 2, being 0 < x < 2; ii) optionally contacting the solid Mg(OR 1) x X 1 2-x obtained in step i) with at least one activating compound selected from the group formed by activating electron donors and metal alkoxide compounds of formula M 1 (OR 2) v-w (OR 3) w or M 2 (OR 2) v-w (R 3) w, to obtain a second intermediate reaction product; wherein: M 1 is a metal selected from the group consisting of Ti, Zr, Hf, Al or Si; M 2 is a metal being Si; v is the valency of M 1 or M 2, w is smaller than v; R 2 and R 3 are each a linear, branched or cyclic hydrocarbyl group independently selected from alkyl, alkenyl, aryl, aralkyl, alkoxycarbonyl or alkylaryl groups, and one or more combinations thereof; wherein said hydrocarbyl group may be substituted or unsubstituted, may contain one or more heteroatoms, and preferably has from 1 to 20 carbon atoms; iii) contacting the first or second intermediate reaction product obtained in step i) or ii) respectively, with a halogen-containing titanium compound and either an activator or an internal electron donor to obtain a third intermediate reaction product; iv) modifying the third intermediate reaction product obtained in step iii) with a modifier having the formula M(p)X p, preferably MX 3, wherein M is a metal selected from the Group 13 metals and transition metals of the IUPAC periodic table of elements, p is the oxidation state of M, and wherein X is a halide, to yield a modified intermediate reaction product; v) contacting said modified intermediate reaction product obtained in step iv) with a halogen-containing titanium compound and in the case that in step iii) an activator was used an internal donor to obtain the procatalyst, wherein preferably step v) is carried out at least two times.