Easy access to Google's email service Gmail is now blocked in China, according to reports.
Using Gmail directly via Google's site has been impossible in the country for some time, but locals had still been able to use third-party apps, such as Microsoft Outlook, to use the service.
However, Google's own data indicates such traffic took a nosedive on Friday and has only slightly recovered since.
The US firm said there were no known issues with its provision of Gmail.
"There's nothing technically wrong on our end," Taj Meadows, a spokesman for Google Asia Pacific, told news agency Associated Press.
The digital rights campaign group, GreatFire.org, was one of the first organisations to flag the fact that internet protocol addresses used to let software access Gmail had become inaccessible in China.
"Those protocols are used in the default email app on iPhone, Microsoft Outlook on PC and many more email clients," it said.
"Chinese users now have no way of accessing Gmail behind the GFW [great firewall]."
Google's data suggests there is still, however, a low level of Gmail use in China.
The Wall Street Journal has also reported that some users had reported Gmail access being restored to their mobile devices.
The Chinese government has neither confirmed nor denied that it was behind fresh restrictions.
"The past two years have seen a consistent tightening of all kinds of censorship on the internet and media," said Jeremy Goldkorn, founder of Beijing-based media tracker Danwei.
"There is an increasingly aggressive attitude towards what they [Beijing] call 'internet sovereignty' and they are confident about talking about internet censorship in positive terms."
Google closed its China office in 2010 following a rocky relationship with the authorities about its handling of censorship.