* Hiroshima High Court overturns lower court ruling from March
* Court says Ikata No.3 reactor not to restart until Sept 2018
* Operator Shikoku Electric's shares plunge more than 10 pct (Adds comment, updates share price)
TOKYO, Dec 13 (Reuters) - A Japanese court on Wednesday ordered Shikoku Electric Power Co not to restart one of its reactors, overturning a lower court decision and throwing into turmoil Japan's protracted return to nuclear power after the Fukushima disaster.
The decision by the High Court in Hiroshima in western Japan has no immediate effect on Shikoku Electric's operations because the reactor in question has been idled for maintenance, but it casts into doubt any restart. Shikoku Electric shares fell more than 8 percent after the ruling and the company canceled plans for a bond sale.
The ruling also marks a victory for Japan's anti-nuclear movement, which has won a number of lawsuits seeking to halt or prevent nuclear operations in recent years, only to see them overturned by more conservative higher courts.
"This is unexpected and goes against the tide of gradual restarts of nuclear plants," said Fumio Matsumoto, a senior fund manager at Dalton Capital (Japan) in Tokyo.
"Despite the progress in nuclear restarts, Japanese power companies face an uncertain future," Matsumoto said.
Other utilities also fell on Wednesday, with Kansai Electric Power Co dropping 3.2 percent and Kyushu Electric Power Co losing 2.3 percent. The Nikkei 225 fell 0.5 percent.
Kansai Electric and Kyushu Electric are the only other two nuclear operators with reactors running.
Just four reactors are currently operating out of 42 commercially viable units. All reactors in Japan had to be relicensed after a new regulator was set up following the Fukushima disaster of 2011.
Residents have lodged injunctions against most nuclear plants across Japan. Lower courts have been increasingly siding with them on safety concerns, but the verdicts are usually overturned in higher courts.
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The court ruling overturns a March decision by Hiroshima District Court to back Shikoku Electric against residents, who want to close down the 890-megawatt No.3 reactor at the power company's Ikata nuclear station.
Shikoku Electric said the ruling was regrettable and that it would file an objection with the high court.
A Shikoku Electric spokesman said the injunction ordered the company not to restart its Ikata No.3 reactor until Sept. 30 next year. The company could face about 3.5 billion yen ($31 million) a month in additional costs, such as for increased use of fossil fuel for power generation, with the delay.
Shikoku had been planning to restart the reactor around Jan. 20, the spokesman said.
The company said it cannot yet estimate any changes to profit forecasts. Court actions against the Ikata plants reactors are also pending in several other courts, it said.
Shikoku Electric also decided to cancel a 10 billion yen bond sale after the ruling, said an official in the company's treasury department. The bonds were to be priced on Thursday.
While Prime Minister Shinzo Abe's government is keen to restore a power source that provided about a third of electricity supply before the meltdowns at Fukushima, the public remains deeply skeptical over industry assurances on safety.
($1 = 113.3100 yen) (Reporting by Osamu Tsukimori, Yoshiyuki Osada and Yasunori Fukui; Writing by Aaron Sheldrick; Editing by Kenneth Maxwell and Tom Hogue)