The tap water supplier at the center of a scandal after excessive levels of a carcinogenic compound were found in its samples has denied a cover-up of the contamination.

The tap water supplier,  the Lanzhou Veolia Water Company, has denied a cover-up of the contamination. [Photo/Xinhua]

Excessive levels of benzene in the water affected more than 2.4 million people in Lanzhou in northwest China's Gansu Province, provincial authorities said on Friday.

The supplier, the Lanzhou Veolia Water Company, collected water samples on April 2 and found abnormal levels of benzene during analysis on Thursday, said Yan Xiaotao, deputy general manager of the Sino-French joint venture.

The excess of benzene was confirmed by further tests at 3 p.m. on Thursday and the company reported the situation to the Lanzhou municipal government, Yan said.

There was no late reporting or cover-up. The government investigation results would have the final say, he told reporters on Monday afternoon.

From 5 p.m. on Thursday to 2 a.m. on Friday, Lanzhou Veolia's tests found benzene levels were 118 to 200 micrograms per liter of tap water in its plants, far higher than the country's limit of 10 micrograms.

The municipal government then warned citizens not to drink tap water from 11 a.m. on Friday to 11 a.m. on Saturday.

China began to adopt compulsory drinking water criteria in July 2012 and the number of indicators increased to 106 from the previous 35. Comprehensive quality tests for tap water are required to be conducted every six months, while daily tests do not include checks for benzene.