Abstract: The present invention relates to a high activity catalyst composition for the (co-)polymerisation of ethylene, optionally with an alpha olefin of 3 to 10 carbon atoms, comprising a catalyst precursor and an organoaluminium cocatalyst, wherein the catalyst precursor consists of:(i) a silica carrier material, having from 0.3 to 1.2 mmoles of OH groups per gram of silica,(ii) a dialkylmagnesium compound of the formula RMgR.sup.1, where R and R.sup.1 are the same or different C.sub.2 -C.sub.12 alkyl groups, in an amount comprised between 0.5 to 1.5 mmoles of dialkylmagnesium per gram of silica,(iii) a tetraalkyl orthosilicate, in which the alkyl groups contain from 2 to 6 carbon atoms, in an amount comprised between 0.2 to 0.8 mmoles per gram of silica,(iv) a chlorinated compound (X) having the formula R.sub.n SiCl.sub.4-n, wherein each R is the same or different and is hydrogen or an alkyl group and n is an integer from 0 to 3, in an amount comprised between 0.