text: Under the Dome explains the social and health costs of pollution, and was watched by more than 100 million people online, sparking debates.
  It was removed just two days after Premier Li Keqiang called pollution a blight on people's lives.
  Mr Li had promised to fight it with all the government's might.
  The environmental issue has dominated the current session of the Chinese parliament, the National People's Congress, in Beijing.
  The newly appointed environmental protection minister, Chen Jining, had praised Under The Dome, telling reporters it should "encourage efforts by individuals to improve air quality".
  But having initially praised the documentary, China's communist leaders now seem to have banned it, the BBC's Michael Bristow reports.
  It appears to tell an inconvenient truth, something the leaders do not want to hear during the current annual meeting of parliament, he adds.
  Standing in front of an audience in a simple white shirt and jeans, Ms Chai speaks plainly throughout the 103-minute video, which features a year-long investigation of China's noxious pollution problem.
  At times, the documentary is deeply personal. Near the start of the documentary, Ms Chai interviews a six-year-old living in the coal-mining province of Shanxi, one of the most polluted places on earth.
  "Have you ever seen stars?" Ms Chai asks. "No," replies the girl.
  "Have you ever seen a blue sky?" "I have seen a sky that's a little bit blue," the girl tells her.
  "But have you ever seen white clouds?" "No," the girl sighs.
  The film watched more than 100 million times
  Under the Dome, a year-long investigation of pollution in China, had garnered more than 100 million views in less than 48 hours.
  Made by renowned investigative journalist Chai Jing and funded with her own money, the film sharply criticises the Chinese state's lax environmental laws.
  As of Saturday, the film was no longer available on popular Chinese mainland video sites.
  A link on the Youku website that previously led to the video now prompts the message, "We're very sorry, Youku was unable to find the page you requested".
  Neither Youku or Ms Chai responded immediately when asked to comment, the Reuters news agency reports.
  China operates the world's most formidable online censorship machine, known as the Great Firewall.
  Some social media users in China voiced frustration at the removal of the film.
  "When will this country be able to face the attitudes of its own people?" wrote one Weibo user, quoted by AFP news agency.
  Another Weibo user quoted by Reuters wrote: "Some people have the power to completely smother Chai Jing's Under the Dome on the internet but don't have the power to smother haze in this country."
  Alvin Lin, the Beijing-based China climate and energy policy director of the US-based environmental group the Natural Resources Defense Council, said: "They've made a big deal about air pollution in the last couple of [legislative meetings]. Under the Dome made it so they really really have to talk about it."
  Willy Lam, a political analyst at the Chinese University in Hong Kong, said: "They are really serious about this except the problem is really entrenched.
  "It is intertwined with all aspects of industry and agriculture and so forth, and it's a really difficult problem to tackle."
  The film is still available on YouTube with English subtitles.
target: The authorities in China have removed from websites a popular documentary which highlights the country's severe pollution problem.