Tencent Holdings Ltd said it was the internet flipping not hacker attacking that caused Monday evening's 40-minute shutdown of one of its websites, QQ.com, portal for China's biggest instant messenger software by client numbers, China News Agency reported.

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The shutdown started at 6:00 pm and it was 40 minutes before the site recovered. About 4 percent of netizens from around the country said they couldn't open the website, according to an online survey conducted by Sina.com.

On one occasion last year, Baidu Inc was attacked by hackers, causing massive records of users being locked out of the site.

China's Internet Network Information Center said the reason for the latest shutdown is still under investigation.