Abstract: A lower &agr;-alkene polymerisation heterogeneous solid catalyst which comprises a hydrated magnesium chloride derived procatalyst, a cocatalyst comprising an organoaluminium compound and a selectivity control agent comprising an ester or ether. The procatalyst comprises a titanium tetrahalide supported on a magnesium chloride ester complex precursor. The internal electron donor ester component of the precursor is generated insitu by reaction of a magnesium chloride alcoholate with an activated carbonyl compound in the mole ratio 0.5-1:10-20. The magnesium chloride alcoholate is formed of hydrated magnesium chloride and an aliphatic alcohol in the mole ratio of 0.5-1:10-20. The mole ratio of the aluminium to the titanium is 10-300:1 and that of the selectivity control agent to titanium is 10-100:1. The process for the preparation of the catalyst comprises mixing a hydrated magnesium chloride derived procatalyst, a cocatalyst and a selectivity control agent.