The Nuclear Regulation Authority will officially prohibit the Monju prototype fast-breeder reactor from resuming operations, dashing the operator's hopes of restarting the facility by the end of next March.

The harsh measure is a setback for the Japan Atomic Energy Agency, a national research institute, and came in response to slack safety checks observed at the Monju reactor in Fukui Prefecture. A written order was to be given to JAEA on Thursday.

The agency is expected to face further scrutiny after 33 of the 55 people working at its laboratory in Tokai, Ibaraki Prefecture, were exposed to radiation in an accident on May 23.