Google's webmail service, Gmail, is now completely blocked in China, as the authorities prevent traffic routed via Hong Kong from reaching its 620 million netizens, experts and Internet monitors said.
"Chinese users now have no way of accessing Gmail behind the GFW," the Internet monitoring group GreatFire.org said in a post on its blog on Monday, in a reference to the complex system of blocks, filters, and human censorship known colloquially as "The Great Firewall," or GFW.
Before Dec. 26, they could still send or receive emails via email clients even though Gmail's web interface is not accessible, the post said.
"Chinese users now have absolutely no way of using Gmail, except using circumvention tools."
Previous disruptions were sporadic, and largely designed to make it look as if Gmail's own servers were unstable, it said.
"GFW has been doing it gradually and now it finally completed the grand mission of completely eliminating Google...in China," the group, which monitors the GFW, said.
Internet performance monitoring company Dyn Research also said on its Twitter account on Sunday that all Gmail traffic via the former British colony is affected by the block.
Dyn's vice-president of analytics Earl Zmijewski told the International Data Group (IDG)'s news service that all Gmail traffic to China through Hong Kong is affected.
Only users who have the software and some technical knowledge will be able to evade the block, he said.
Sharp fall in traffic
According to Zmijewski, the traffic is being censored at the level of IP addresses, a method often used by regimes to block content to a particular region, IDG reported.
Meanwhile, Google's own measurement of China-bound traffic showed a sharp fall starting on Friday, the company's real-time traffic measurement tool revealed.
Government censors last blocked Gmail temporarily ahead of the 25th anniversary of the June 4, 1989 crackdown on pro-democracy protests in Beijing's Tiananmen Square.
China has blocked access to Google search, YouTube, and Picasa photographs continually since 2009, according to Google's Transparency Report.
The GFW already ensures that popular foreign websites like Facebook, Twitter, and YouTube are hard to access for the majority of Chinese users.
Google left China in 2010 after a showdown with the government over Internet controls, and currently redirects Chinese language users from the mainland to a search site run from its Hong Kong-based servers.
Gmail has long been accessible, however, and is viewed by rights activists and dissidents as potentially more secure than other mail services.
Shenzhen-based Internet entrepreneur Zhang Jinjun said the authorities appear determined to cut off the Chinese Internet from the outside world.
"A lot of users think that Gmail has better security features, so they use it more than the others," Zhang told RFA on Monday.
"The servers are outside mainland China, which is a good thing for the privacy of individual users."
He said the block will throw large numbers of people inside China into disarray.
"This will cause huge inconvenience for a lot of users, regardless of whether they are commercial or individual users," Zhang said.
"This seems to run counter to the authorities' declared strategy of greater integration."
Chinese free speech advocates slammed the move, mostly via Twitter, as the homegrown equivalents are closely censored by the ruling Chinese Communist Party.
Investigative journalist Fang Zhouzi tweeted: "2014 will go down in history as the year that we had more Internet users than the rest of the world, and in which all of Google's services including the most secure Gmail were shut off behind the Great Firewall."
And Chinese free speech advocate Wen Yunchao, known by his online nickname Beifeng, wrote: "For them to block Gmail shows that the authorities aren't going to give any quarter at all."
Reported by Xin Lin for RFA's Mandarin Service. Translated and written in English by Luisetta Mudie.