* FSA denies media report it's considering capital surcharge
* Shares of Japan's top three banks gain after announcement (Adds background, details)
TOKYO, Oct 6 (Reuters) - Japan is not considering imposing a capital surcharge on big banks on top of Basel III international standards, an official at Japan's financial regulator said on Wednesday, denying a media report it was mulling such a move.
Shares of Mizuho Financial Group surged after the announcement and gained more than 8 percent as the bank, Japan's second biggest and the least capitalised among the country's top lenders, is seen as the most vulnerable to tougher capital requirements.
Bloomberg reported earlier on Wednesday that Japan's Financial Services Agency was considering making top banks hold more capital beyond the Basel III standards set out three weeks ago, citing a person with direct knowledge of the matter.
"There is no such fact that the Financial Services Agency is considering a surcharge on megabanks' required capital," Atsushi Mimura, director at the FSA's Office of International Affairs, told a hastily arranged briefing.
"(The media report) did not reflect the truth and we judged that we needed to send a correct message to the market," he said.
The main requirement of the Basel III rules, to come into force by 2019, is to have a top-quality capital ratio of 7 percent. Regulators have also been discussing proposals to regulate systemically important financial institutions to be presented to a G20 summit in Seoul in November.
Late last month, Financial Stability Board chief Mario Draghi said there had been "unanimous" agreement among regulators on the need to strengthen big banks beyond the levels of the Basel III capital requirements.
But the Japanese regulator has voiced caution towards such a capital surcharge being implemented across the board globally.
Fears about more burdensome capital rules rose after Switzerland tightened the reins on UBS and Credit Suisse, telling them earlier this week to hold more capital than international rivals to prevent a bank failure from crippling the country.
Shares of Mizuho jumped 8.6 percent to 126 yen. Mitsubishi UFJ Financial Group rose 2.6 percent to 401 yen and Sumitomo Mitsui Financial Group rose 1.8 percent to 2,494 yen. (Reporting by Taiga Uranaka and Nobuhiro Kubo; Editing by Chris Gallagher)