Process comprising polymerizing in a loop reactor of a continuous tubular construction an olefin monomer optionally together with an olefin comonomer in the presence of a polymerization catalyst in a diluent to produce a slurry comprising solid particulate olefin polymer and the diluent, wherein the average internal diameter of at least 50% of the total length of the continuous tubular loop reactor is at least 700 mm wherein the HMW polymer is produced in a reactor upstream of the LMW polymer reactor and the ratio of the average internal diameter of the HMW reactor to the average internal diameter of the LMW reactor is between 0.8 and 1.4.