China's Internet firm Tencent on Wednesday said it is shutting down its popular instant messaging service QQ on computers installed with anti-virus software run by the company's rival Qihoo 360, as a war between the two software giants escalated over the course of the past two months.

"A new tag-on service run by 360 Safe has affected the normal functioning of QQ. With 360 Safe installed, we can not guarantee the safety of our service," Tencent said in a statement popping out on screens of millions of QQ and 360 Safe users. "To avoid our clients' computer desktops being turned into a battlefield, we have made this 'hard' decision."

A Tencent official told Xinhua that the company felt "sorry" for the inconvenience brought to QQ users, but the decision will not be changed unless Qihoo 360 removes the tag-on service and stops its "malicious slander" against Tencent.

Qi Xiangdong, president of Qihoo 360, said Tencent's retaliatory move was disastrous for China's Internet users, as it might lead to an unprecedented outbreak of viruses.

"Our 360 Safe services are eliminating about 60 million Trojan horses daily for Chinese Internet users. If they uninstall 360 Safe, the scale of these virus attacks will be beyond imagination, " Qi said.

According to the companies' websites, the number of active QQ users in China has reached 1 billion, while Qihoo 360 also claims to have had at least 300 million users prior to June 2010.

The war between Tencent and Qihoo 360 started in late September when Qihoo 360 alleged QQ was leaking users' private data and offered a new service on 360 Safe to prevent privacy leaks. Tencent has since accused Qihoo 360 of slander and business foul play.

Qi said the Ministry of Industry and Information Technology and the Ministry of Public Security have joined in consulting regarding the dispute.

The escalated business war has also triggered a public outcry. Over 78.1 percent of Internet users participating in an on-line survey conducted by the country's most popular portal, Sina.com, said Tencent and Qihoo 360 fought for their own interests but both forgot the interests of their clients.

"At this stage, when the populace is angry, neither company wins the war," said Liu Xingliang, an Internet commentator.