The producers of spirulina products are facing a credibility crisis after a report claimed excessive lead was found in the popular dietary supplement sold in China.

Samples of eight popular brands of spirulina products were purchased from drug stores, supermarkets and shopping centers in Beijing, Tianjin, and Hebei province.

Six of the eight Chinese and foreign brands tested contained excessive levels of lead, the report said. The tests were conducted at different organizations and qualified by China's Certification and Accreditation Administration.

The six brands included U-Ways, Green-A and Cont-healthy, Xinhua reported on Wednesday.

Cont-healthy, a US brand, exceeded the acceptable limit by 820 percent, it said.

Normally, China does not allow a kilogram of a food product to contain more than 0.5 milligram of lead. Excessive lead can harm the digestive, nervous and reproductive systems and cause stomachaches, anemia and convulsions.

Representatives of Cont-healthy were not available for comment.

Green-A said on Wednesday on its website that all of its products passed the required tests.

Shares of Shenzhen-listed Guangdong By-health Biotechnology Co were suspended from trading on Thursday after Xinhua reported that the lead amount in its samples of spirulina tablets is 100 percent more than the country's standard.

Trading will resume after the relevant facts are verified, a notice said. The company's products passed a test in March by an organization appointed by the State Food and Drug Administration, the company said in an announcement posted on its website on Wednesday.

Spirulina products are popular nutritional supplements in China, and they are believed to be rich in protein, vitamins and minerals.

The Xinhua report claimed that substandard spirulina powder was the cause of excessive lead, adding that some agents helped manufacturers obtain official permits to sell unqualified products as qualified ones.

Those agents fabricated samples and bribed drug inspectors, the report said.

"It is quite common that some manufacturers use substandard spirulina powder just to slash the costs for bigger profits," an industry insider in Yunnan, a major producer of spirulina, said on condition of anonymity.

The State Food and Drug Administration has ordered a recall of all tainted spirulina products, Tong Min, an official from the administration, was quoted by Xinhua as saying.

As of Thursday afternoon, all spirulina products from the six brands had been removed from Taobao, a popular online shopping website, Suo Chao, a staff member of public relations at Taobao, said.

The case is the latest food safety scandal in the world's most populous country.

"It's astonishing," said Song Qingxia, a Beijing resident. "I will earnestly think of sending my 65-year-old father to hospital for a checkup. He has been eating Green-A for two years."