Abstract: The invention relates to a method for the preparation of a catalyst suitable for the polymerisation of an olefin by contacting a magnesium compound with a halogenized group 4 or 5 metal compound, wherein the magnesium compound is obtained by the reaction of a solution of an organomagnesium compound with a silicon mixture or compound, characterized in that (a) the organomagnesium compound solution is obtained by contacting metallic magnesium Mg with an aromatic halide RX and an ether R0, wherein R is an aromatic group containing 6 to 20 carbons and X is a halide, and (b) the silicon mixture or compound is the product obtained by mixing or reacting a hydrocarbyl halide silane with an alkoxy group or aryloxy group containing silane compound.