Process for polymerising an olefin monomer in at least one continuous tubular loop reactor of a multiple reactor system, optionally together with an olefin comonomer, in the presence of a polymerisation catalyst in a diluent, to produce a slurry containing solid particulate olefin polymer and diluent. The average internal diameter of at least 50% of the total length of the continuous tubular loop reactor is at least 700 mm. A high molecular weight (HMW) polymer is made in a first reactor and a low molecular weight (LMW) polymer is made in a second reactor, the first (HMW) reactor having a space time yield (defined as production of polymer in kg/h per unit volume of reactor) greater than 100 kg/m3/h, and the ratio of space time yield in the first (HMW) reactor to the second (LMW) reactor is greater than 1.