A delay would be a further hitch in the protracted reboot of Japan's nuclear sector, which was shut down in the wake of the Fukushima disaster in 2011. The government and industry want reactors restarted to cut electricity bills, but swathes of the public oppose returning to atomic energy.

Four reactors are operating out of 42 commercially viable units, and Kyushu Electric has been planning to restart two of its reactors at its Genkai plant in southern Japan by next March.

A Kyushu Electric spokesman on Wednesday told Reuters that the firm had not yet changed the schedule for the Genkai restart, but added that the utility had told the country's atomic regulator in mid-November that checks on the use of Kobe Steel products would take about a month.

A delay would mark the first direct impact on reactor restarts from the Kobe Steel scandal, raising worries over similar delays in restarts at other nuclear plants, the Nikkei said. The paper cited a senior company official as the source for its information on the possible delay in the Kyushu restart.

Japan's third-largest steelmaker, which supplies producers of cars, planes, trains and other products across the world, said in October that about 500 of its customers had received products with falsified specifications. The company is also a supplier to the nuclear industry, providing casings for uranium fuel rods and for spent fuel cooling units.

The No.3 and No.4 reactors at Genkai plant in southern Japan have met the regulator's safety requirements imposed after the Fukushima disaster, and the company had been aiming to restart the No.3 reactor in January and No.4 unit in March.

Checks by nuclear operators have so far found that some Kobe Steel parts are used at their nuclear plants, but that there are no safety issues as the supplied products were not made at factories that engaged in fabrication.

Checks are continuing and utilities are prioritising operating units and those in final stages before restart.

Kansai Electric Power is planning to restart two units at its Ohi plant west of Tokyo by March and is making checks to see whether they have parts supplied by Kobe Steel that have falsified data.

The utility so far has no plan to delay the restart process, a spokesman told Reuters on Wednesday.

(Reporting by Osamu Tsukimori; Writing by Aaron Sheldrick; Editing by Joseph Radford)