A process for the polymerisation of olefins in the gas phase is carried out in the presence of a supported transition metal catalyst. The process involves a prepolymerisation step which may be performed in-situ and in particular the prepolymer may be prepared in the dry phase. The catalyst may for example comprise a constrained geometry transition metal complex supported on silica and used in the presence of an activator. The prepolymerisation step enables the activity of the catalyst to be improved.