Technology giants such as Google LLC, Apple Inc. and Inc. came under pressure Wednesday as Japan's antitrust watchdog noted opaque practices that could disadvantage vendors on their e-commerce sites or smartphone app stores.

Some 50 to 90 percent of such vendors said their contracts have been changed in Japan and abroad without prior negotiations, a survey by the Japan Fair Trade Commission found, fueling concerns that the tech groups are abusing their dominant position against smaller companies using their platforms.

The poll showed that 93.2 percent of suppliers on the e-commerce site run by Japan's Rakuten Inc. said they have experienced unilateral contract changes. Similar responses were collected from 81.4 percent of vendors for Apple, 73.8 percent for Google and 72.8 percent for Amazon.

The internet survey was conducted between Feb. 27 and March 26, collecting 867 responses. The FTC said it will continue its probe, including the holding of interviews with the large tech firms known in Japan as "platformers."

The commission has already raided offices in Japan of some platformers such as Amazon, which allegedly violated antitrust laws by having vendors shoulder part of the costs to cover discounts it applied to goods sold online.

The Japanese government is expected to compile soon new rules to ensure more transparent transactions between platformers and vendors.