Chinese internet users have had serious problems using Google's international search service, prompting speculation it is being disrupted by government censors.
Users in many Chinese cities have been able to access Google.com only intermittently in recent days.
The problems have not affected Google.cn, the US company's China-based search service, which is censored.
A Google spokesperson said on Wednesday: "We have heard reports of users in China experiencing problems in accessing Google.com. We are investigating this matter."
Beijing does not usually comment on which websites it blocks. But the problems suffered by Google are similar to those experienced by other websites targeted by censors.
China has disrupted Google's service before, blocking it completely in 2002 and redirecting visitors to China-based websites including Baidu.com. Beijing lifted the block amid protests by internet users and international publicity.
The current problems are the worst in recent years, prompting suggestions that Google's introduction this year of its censored service had emboldened Beijing to act against the unfiltered international version.
Reporters Without Borders, the media freedom watchdog, said: "It was only to be expected that Google.com would be gradually sidelined after the censored version was launched in January.
"Google has definitively joined the club of western companies that comply with online censorship in China. It is deplorable that Chinese internet users are forced to wage a technological war against censorship in order to access banned content."
Beijing blocks thousands of overseas websites and monitors internet cafés round the clock.
The censorship system was blamed by many users for problems accessing Microsoft's Hotmail service just weeks after President Hu Jintao visited Bill Gates' Seattle home.
Reuters this week quoted Sergey Brin, Google co-founder and president, as saying that only 1 per cent of Chinese users used Google.cn, with most sticking with Google.com.
