Hundreds of millions of China's web users must decide whether they want to keep using the country's most popular chatting tool, or its top Internet security tool - because they can't have both.

In the latest twist of a bitter battle between two of China's biggest dot.com names, Tencent Inc (QQ), which has more than 1 billion instant message user accounts, announced last night that it is withdrawing QQ services from 300 million users of Qihoo's 360 Internet security services.

In a statement, Shenzhen-based Tencent claimed that Qihoo's software had "damaged the security setup of QQ and, therefore, threatened the account safety of QQ users."

Responding, Beijing-based Qihoo advised users to stop opening QQ for three days and switch to other instant message tools, such as Microsoft's MSN and China Mobile's Fetion.

In the longer term, Qihoo suggested QQ users transfer to Web QQ, a simplified QQ service without security and other functionality.

Qihoo has claimed that Tencent was scanning users' documents through QQ, and last week launched a tool to block QQ plug-ins.

Both firms deny claims they scanned users' private information or threatened account safety, and have sent software code to regulators. No results were available last night.

At 10pm, Qihoo said it had stopped providing downloads for a tool called 360 Koukou Guard (its name sounds like QQ in Chinese) due to "technical conflict with QQ and possible disturbance for users."

Qihoo released 360 Koukou Guard last week. It deletes unnecessary documents, blocks plug-ins and filters popup ads produced by QQ.

"Tencent seems to have over-reacted (towards 360's move) and each firm should not deprive users of the right to choose both products," said Lu Bowang, president of the China IntelliConsulting Corporation.

Some analysts believe most users will choose to keep their QQ account and uninstall Qihoo 360, as users are usually more loyal to online social tools than security services.

Three industry leaders Tencent, Baidu.com Inc and Kingsoft Corp- have published a joint statement accusing Qihoo of unfair play because its application warns users that their software poses a security threat.

The battle began in February when Tencent encouraged users to install QQ Doctor, which has similar functionality to Qihoo 360. It is widely expected to put pressure on 360.

Kingsoft, Kaspersky, Rising and other software makers, which sell anti-virus software, also spoke out against Qihoo's 360, which is free.

Qihoo retorted that these companies were "jointly committing evil" at the price of users' interests and privacy.

China has the world's biggest online population with more than 400 million Internet users.