With peak summer power demand looming, and savings on fuel costs elsewhere in prospect, the Osaka-based utility aims to restart the Mihama No.3 reactor in late June, the first restart of a reactor that is more than 40 years old in Japan since safety standards were upgraded following the Fukushima disaster.

But the company said in a filing on Wednesday that it won't be able to implement tighter anti-terrorism standards ordered for all nuclear plants in Japan before the Oct. 25 deadline for it to do so at Mihama No. 3, meaning the reactor will be taken off line again before then.

Kansai Electric received regulatory approval last month to restart three reactors that are more than 40 years old, Mihama No.3 and the No. 1 and No.2 reactors at its Takahama nuclear power station. All have been idle since Japan effectively idled its whole nuclear power sector following the Fukushima crisis.

But the deadline for anti-terrorism upgrades at Takahama No.1 and No.2 is June 9, it said, meaning it won't be able to restart those two reactors.

"The timing of completing anti-terrorism upgrades for the three reactors is not known," a company spokesman said on Thursday.

The utility currently has three other nuclear reactors on line.

(Reporting by Yuka Obayashi; Editing by Kenneth Maxwell)