The European Commission is co-operating with the Japanese authorities in an investigation of Intel for possible antitrust violations.
Japan's Fair Trade Commission warned Intel, the world's largest microchip maker, about unfair business practices it said were stifling competition in the Japanese chip market.
The Japanese antitrust watchdog said Intel's Japanese unit offered rebates to five Japanese personal computer makers that agreed either not to buy or to limit their purchases of chips from Intel's rivals, namely Advanced Micro Devices and Transmet .
Intel disputed the warning, which did not come with a fine, saying it acted fairly and legally.
Last week Ireland withdrew €170 million of promised aid to Intel to avert an investigation by the European Commission.
The commission believed the aid, aimed at helping build a cutting-edge microchip plant, could violate EU rules limiting state aid to companies.