A worker at Japan’s nuclear safety agency lost a work-issued phone containing sensitive information during a personal trip to China, local media reported.
The device held confidential contact details of staff involved in nuclear security at the Nuclear Regulation Authority.
The agency said it could not confirm whether the information had been accessed or leaked.
The employee is believed to have lost the phone on 3 November during a security check at an airport in Shanghai.
He realised the phone was missing three days later and was unable to recover it despite contacting airport authorities.
The NRA provides work smartphones to some staff to ensure rapid responses during emergencies.
The affected department is responsible for protecting nuclear materials from threats such as theft and terrorism.
The incident comes as Japan works to restart its nuclear energy programme after years of shutdowns.
All nuclear reactors were halted in 2011 following the Fukushima disaster triggered by a major earthquake and tsunami.
The NRA was created after Fukushima to oversee nuclear safety and reactor restarts.