Great Wall Motor's Haval H6 was the best-selling SUV for years, but that spot has been taken by BYD's Song family of models.

Chinese fuel-car giant Great Wall Motor is making its conflict with local new energy vehicle (NEV) giant BYD (OTCMKTS: BYDDY) public, as the competition between the two grows fiercer.

Great Wall Motor has filed a report with Chinese regulators against BYD, alleging that the latter's Qin Plus DM-i and Song Plus DM-i use normal-pressure fuel tanks and are suspected of having substandard evaporative pollutant emissions, according to a statement today.

Great Wall Motor filed the report with China's Ministry of Ecology and Environment, the State Administration of Market Supervision and Administration, and the Ministry of Industry and Information Technology on April 11, according to the statement.

Under Chinese law, environmental regulators are supposed to conduct a preliminary review of suspected violations and decide whether to open a case, according to the statement.

Great Wall Motor is now closely monitoring regulators' progress in handling the report, the statement said.

Great Wall Motor was once the top-selling SUV company in China, with its Haval H6 being the best-selling SUV for many years.

But in the past few years, BYD has risen to the top at a time of rapid growth in China's NEV industry, with its Song family of models often becoming the new No. 1 SUV seller.

From January to April, the BYD Song family sold 174,422 units, up 76.5 percent from 98,809 units in the same period last year, making it the best-selling SUV in China during that period, according to the China Passenger Car Association (CPCA).

The Haval H6 sold 63,682 units from January to April, down 25.9 percent from 85,986 units in the same period last year, placing it fifth in the best-selling SUV retail sales rankings.

Great Wall Motor is trying to recapture lost share in the SUV market with new model launches, making the Xiaolong line of plug-in hybrid SUVs available on May 16, offering the Haval Xiaolong as well as the Haval Xiaolong Max models.

The Haval Xiaolong is a compact SUV aimed at the Song Pro DM-i series, while the Haval Xiaolong Max is a mid-size SUV aimed at the Song Plus DM-i series.

Later today, BYD will officially launch the 2023 Song Pro DM-i in China, which is expected to bring the price down from the previously available version.

Full CPCA rankings: Top-selling models and automakers in China in Apr