(Adds company confirmation, details)

TOKYO, July 18 (Reuters) - Japan's Tokyo Electric Power Co. (TEPCO) <9501.T> underreported the amount of radiation in contaminated water that leaked into the ocean after its nuclear power plant -- the world's largest -- was hit by an earthquake this week, the company said on Wednesday.

TEPCO said the underreporting was due to a miscalculation but the amount was still below national safety levels.

The company said Monday's quake sparked a leak of water contaminated with 90,000 becquerels of radiation, compared with the figure of 60,000 becquerels it had previously given.

TEPCO had said about 1,200 litres of radioactive water leaked into the sea from its Kashiwazaki-Kariwa nuclear plant, where a small leak of radiation into the atmosphere and a fire were also sparked by the tremor.

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