The main aim of the JET KN3 gamma-ray cameras upgrade is the design of appropriate neutron/gamma-ray filters ("neutron attenuators"). Using the attenuation factor as the main design parameter a set of three neutron attenuators of different shape and attenuation length have been designed for the horizontal and vertical cameras. Pure light water was chosen as the attenuating material for the JET gamma-ray cameras. The operation of the KN3 neutron attenuators is controlled by an electro-pneumatic system. A full-scale prototype of the vertical camera neutron attenuator was constructed and tested. The mechanical behaviour of the attenuator structure subject to the forces and torques produced by the JET disruptions was analysed by means of the finite element analysis method. The radiation performance of the KN3 neutron attenuators as well as the response of the gamma-ray detectors have been addressed by means of neutron and gamma-ray transport calculations.