The present invention deals with a process of polymerising at least one olefin in a fluidised bed in a fluidised bed polymerisation reactor comprising a top zone, a middle zone in direct contact and below, a bottom zone in direct contact with and below the middle zone and wherein the reactor does not comprise a fluidisation grid. The process comprises passing a stream comprising the fluidisation gas and polymer particles into a separation step and withdrawing a stream comprising the polymer particles from the separation step and returning it to the polymerisation reactor. The process comprises adding a support gas stream to the stream comprising the polymer particles downstream of the separation step.