Google's Gmail, the world's most widely used e-mail service, has been almost entirely blocked in China for the last three days.
Google's Transparency Report shows a dramatic drop in Gmail traffic to China starting Friday.
Google declined to comment on the reasons behind the drop in traffic. "We've checked and there's nothing wrong on our end," a Google spokeswoman told CNET.
A spokesman from the Consulate General of China's office in New York couldn't immediately be reached for comment.
Chinese Foreign Ministry spokeswoman Hua Chunying told Reuters that she did not know anything about Gmail being blocked.
Google has had issues operating in China for years, tracing back to its clash with the country after refusing to censor its Internet search results in 2010. Its services have worked inconsistently after it moved its operations from mainland China to Hong Kong.
In June, China blocked the use of Gmail through browsers ahead of the 25th anniversary of the Tiananmen Square massacre, in which the government violently cracked down on pro-democracy demonstrators. However, there was a loophole that allowed people to continue to access Gmail through Microsoft's Outlook or Apple's email client. That loophole has apparently been closed.