A catalytic ingredient for use in the polymerization of olefins comprises a carrier comprising a magnesium compound precipitated from solution and a catalytic component supported on the carrier and which is a titanium halide, vanadyl halide or vanadium halide, the catalyst being obtained by a process which comprises the steps of:

(A) mixing (a) one or more magnesium compounds with (c) a saturated or unsaturated monohydric alcohol or a polyhydric alcohol, for reaction in solution in the presence of (b) carbon dioxide in an inert hydrocarbon solvent, to obtain component (A);
(B) subjecting component (A) to mixing and reaction with (d) a titanium halide and/or a vanadyl halide and/or a vanadium halide and/or a silane halide together with (e) a boron compound, thereby giving solid product (I);
(C) reacting solid product (I) with (f) a cyclic ether thereby causing dissolution and re-precipitation to give solid product (II); and
(D) subjecting solid product (II) to further reaction with (g) component (B) consisting of a titanium halide and/or a vanadyl halide and/or a vanadium halide as defined for (d) above, thereby to give solid product (III): followed either by further reaction with a mixture of component (B) and (h) an electron donor or by reaction of (g) with the solid product (III) obtained by the reaction between the solid product (II) and (h), thereby obtaining solid product (IV) for use as the catalytic component.

In modifications, the boron compound (e) may be replaced by a silane and/or siloxane compound; and/or an alcohol or polyhydric alcohol may be used in step (C).