South Korea’s telecommunications regulator said on Tuesday it will launch an official fact-finding investigation into the app stores of Google, Apple, and ONE store next week, saying that the companies may be in violation of the country’s in-app payment laws.

The Korea Communications Commission (KCC) said it will launch the probe into the three markets — Google Play, App Store, and ONE store — on August 16, following provisional inspections of the stores since May 17 over potential violations of the country’s revised Telecommunications Business Act.

The KCC determined that the companies were potentially in violation of the law by enforcing certain in-app payment methods and refusing developers who use external payment methods to register and renew their apps on their markets, reports Yonhap news agency.

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ONE store is a homegrown app market company controlled by South Korea’s SK Group.

Google recently required all app developers selling digital goods and services on the Google Play store to use its billing system and to remove external payment links, prompting many South Korean companies to raise charges for paid content services sold on Google’s store.

Google’s move has prompted the Korea Publishers Association to file a complaint with the KCC, accusing the U.S. tech giant of violating the law and requesting a probe into the billing policy.

Last year, the South Korean National Assembly passed the law that bans app store operators from forcing in-app payment systems on developers, making South Korea the first country in the world to introduce such curbs on in-app billing policies of global tech giants.

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Both Google and Apple app stores are also under scrutiny from the Indian regulator the Competition Commission of India (CCI) over in-app policies and developers’ fees.