Beijing has added yet another brick to its Great Firewall, tightening access to Google's email service in the latest phase of a censorship campaign that has left Chinese citizens unable to access huge swaths of the Internet.
Other Google (GOOG) products, including Search, Sites and Picasa, have been similarly targeted by Chinese government agencies that govern Internet and social media content.
With Gmail access now severely restricted, Google's suite of services are largely blacked out in China.
Some Gmail users in China reported on Twitter Tuesday that service had been restored. But Google's own data still shows that fewer than 20% of people in China can access their Gmail.
The outcry over the latest blockage was swift and angry. Business travelers complained they will no longer be able to access email while in China without jumping through hoops. Their Chinese counterparts complained that it will now be more difficult to conduct business internationally.
And Google is hardly alone.
Related: Banned! 8 things you won't find in China
Access to Twitter (TWTR), Facebook (FB) and YouTube is blocked in China. During recent pro-democracy protests in Hong Kong, the Facebook-owned photo sharing app Instagram was blacked out on the Mainland.
Taken together, the restrictions constitute the world's largest -- and most effective -- state-sponsored censorship program. The effort, officially called "Golden Shield," is more than a decade old.
The program allows Beijing to restrict content it deems sensitive (on democracy, Tibet or the Uighur ethnic group, for example). Thousands of websites are blocked outright, and Chinese citizens that offend authorities can face judicial consequences.
Related: China lifts ban on Facebook for select few
The effort has left Chinese Internet users with a World Wide Web that bears little resemblance to the uncensored Internet.
Instead of using Google to perform searches, most Chinese use a homegrown alternative called Baidu (BIDU). Instead of posting messages on Facebook or Twitter, Chinese users are pushed to Weibo.
And these domestic services are in turn heavily censored.
Offensive content -- even a joke about the Communist Party -- will quickly be removed from circulation. Censors monitor activity 24/7, forcing Chinese users to deploy homophones and puns in an effort to mask their intent. Among the most popular of these is "Grass Mud Horse," a homophone for an obscenity involving one's mother that doubles as an anti-censorship meme.
Related: Chinese smartphone maker valued at $45 billion
For U.S. companies hoping to do business in the world's second largest economy, Beijing's approach presents a series of tough choices.
Companies that resist Beijing's censorship -- as Google has done -- are often punished as a result. Of major U.S. social media platforms, only LinkedIn (LNKD) has been allowed to operate in China -- and only after it agreed to block content. For example, it took down posts earlier this year related to the 1989 Tiananmen Square crackdown.
China is unlikely to ease its restrictions in the near-term. Beijing often describes what is known colloquially as the "Great Firewall" as a critical national security tool.
"I can choose who will be a guest in my home," China's top Internet regulator Lu Wei said earlier this year.
The nationalist-leaning Global Times offered the security justification in an editorial published Tuesday.
"If the China side indeed blocked Gmail, the decision must have been prompted by newly emerged security reasons," the paper said. "If that is the case, Gmail users need to accept the reality of Gmail being suspended in China."
