China is investigating its two telecommunications giants, China Telecom and China Unicom, over their suspected monopolistic practices in the broadband access business, the country's top economic planner said Wednesday.

The National Development and Reform Commission (NDRC)'s probe into the two operators is the first anti-monopoly case involving large enterprises since China implemented its first anti-monopoly law in 2008.

China Telecom and China Unicom account for 90 percent of the country's broadband business and have formed a monopoly in the market, said an official with the the NDRC, who declined to be named.

The two companies would face penalties of up to 10 percent of their annual business revenues if they were found guilty of monopoly, said Li Qing, deputy head of the NDRC's price supervision and anti-monopoly department.

Revenues from broadband business of China Telecom amounts to around 50 billion yuan (about 7.9 billion U.S. dollars) a year while that of China Unicom nears 30 billion yuan a year, Li said.

Results of the investigation are expected to come out soon, said Li.

China has the world's largest Internet population, with the number of Internet users reaching 485 million by the end of June this year, figures from the China Internet Network Information Center show.

However, China's average broadband speed only ranks 71th in the world while average expenses are three to four times the average level of developed countries, said a report released in July by the Advisory Committee for State Informatization, an advisory body to the Chinese government.

Li said China Telecom and China Unicom did not fully integrate their networks and hence increased access costs and slowed down Internet speeds.

She also noted that the two companies were found using their market dominance to practise price discrimination.

China added 23.6 million broadband Internet users in the January-September period, bringing the total number to 149.9 million, according to data from the Ministry of Industry and Information Technology.

As of the end of September, China Telecom had 73.7 million broadband users and China Unicom had 54.5 million, combining to account for 85.5 percent of the national total, according to the two companies' quarterly reports.

Chinese law bans dominant market players from fixing prices. A single party who has more than half of a market's share is considered dominant.

Two parties who have more than two thirds of a market's share and three parties whose market shares exceed three-fourths are also considered dominant.

China Telecom saw its shares traded in Hong Kong fall 0.41 percent on Wednesday. China Unicom shares dipped 2.48 percent in Hong Kong and its shares in Shanghai lost 2.84 percent.