"Imagine if Gmail users might not get through to Chinese clients. Many people outside China might be forced to switch away from Gmail."
Google's own Transparency Report, which shows real-time traffic to Google services, displayed a sharp drop-off in traffic to Gmail from China on Friday.
"We've checked and there's nothing wrong on our end," a Singapore-based spokesman for Google said in an email.
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Almost all of Google's services have been heavily disrupted in China since June this year, but until last week Gmail users could still access emails downloaded via protocols like IMAP, SMTP and POP3. These had let people communicate using Gmail on apps like the Apple iPhone's Mail and Microsoft Outlook.
China maintains tight control over the internet, nipping in the bud any signs of dissent or challenges to the ruling Communist Party's leadership.
The country is host to the world's most sophisticated internet censorship mechanism, known as the Great Firewall of China. Critics say China has stepped up its disruption of foreign online services like Google over the past year to create an internet cut off from the rest of the world.
The Google disruption began in the run-up to the 25th anniversary of the government's bloody crackdown on pro-democracy demonstrators around Beijing's Tiananmen Square on June 4, 1989.
Gmail's setback could make email communication difficult for companies operating in China which use Google's Gmail for their corporate email system, said GreatFire.
Chinese Foreign Ministry spokeswoman Hua Chunying said she did not know anything about Gmail being blocked, adding that the government was committed to providing a good business environment for foreign investors.
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"China has consistently had a welcoming and supportive attitude towards foreign investors doing legitimate business here," she said. "We will, as always, provide an open, transparent and good environment for foreign companies in China."
One popular way for companies and people to get around China's internet censorship is to use a Virtual Private Network (VPN) which allows unhindered access to blocked sites and services.
"It's becoming harder and harder to connect and do work in China when services like Gmail are being blocked," said Zach Smith, a Beijing-based digital products manager at City Weekend magazine. "Using a VPN seems to be the only answer to doing anything these days online in China."