The use of fuel oil is likely the first to be curbed as it is the most expensive substitute and is being burned in older units that had been mothballed until being brought back online after Fukushima.

Demand for coal and liquefied natural gas, which has hit successive records as nuclear plants were shut down, will likely remain high, utilities and analysts have said.

Kyushu Electric's Sendai reactors, commissioned around 30 years ago, received initial safety clearance in September.

Two more rounds of checks are ongoing and, while there have been hitches, the June start date looks feasible for inspectors to finish their job, said another source.

However, there is a chance restarts could be delayed as the units have been shut for more than three years and may need further maintenance.