Abstract: The present invention relates to a polymerisation reactor for continuous polymerisation, the reactor being constructed in two stages and comprising a prepolymerisation stage which is configured as a stirred vessel or as a loop-type bubble column and also a main polymerisation stage which is configured as a tubular reactor. Furthermore, the present invention relates to a method for the production of biodegradable polyester, in particular polylactide, the reactor according to the invention being used. Likewise, the invention relates to a polymerisation device which comprises further components in addition to the polymerisation reactor.