A method for in-line leak testing of blow moulded containers wherein a conveyor line is fed with a continuous stream of blow moulded containers from a blow moulding machine, at least one measuring head is temporarily connected to a first blow moulded container. The measuring head is moved in order to follow the movement of said first blow moulded container which is pressurized by a source of pressurized gas, the source of pressurized gas is disconnected, said at least one measuring head is used to measure the pressure decay therein. The pressure decay is compared with stored data for the pressure decay of acceptable containers and an accept or non-accept signal is generated which implies proceeding or disposal of said first blow moulded container. The measuring head is moved against the moving direction of the conveyor line to a subsequent blow moulded container, and the process is repeated.