China appears to have banned a number of foreign websites, including the BBC's Chinese language news site and Voice of America in Chinese.
The sites had been unblocked after journalists attending the Beijing Olympics complained that the government was censoring sites deemed sensitive.
The BBC expressed disappointment at the apparent reinstatement of the ban.
But a Chinese government spokesman told journalists that some sites contained content that violated Chinese law.
Among the other sites blocked are Asiaweek, Reporters Without Borders and some Hong Kong and Taiwan sites.
China imposes strict controls on the dissemination of information through the web, employing teams of people to remove sensitive content, police bloggers and remove access to certain sites.
In a news conference, foreign ministry spokesman Liu Jianchao refused to confirm that the government was behind the censorship.
But he said some websites violated Chinese law.
"For instance, if a website refers to 'two Chinas' or refers to mainland China and Taiwan as two independent regions, we believe that violates China's anti-secession law, as well as other laws," he said, according to Reuters news agency.
He urged the websites to "comply with China's concerns and not do things contrary to Chinese law".
Mr Liu would not comment on why websites had been temporarily unblocked during the Olympic Games, or comment on the legal process followed to approve the blocking of sites.
In a statement, the BBC said it was disappointed that Chinese-speaking audiences in China were denied access to BBCChinese.com.
It said that except during the 2008 Games, the website had been blocked since its inception nearly a decade ago, and Mandarin radio broadcasts had been "subject to persistent frequency interference for decades".
"It has been a source of great regret that audiences in China are unable to access BBCChinese.com as the rest of the world can," the statement said.
Tough year ahead
Just before the Olympics, foreign journalists complained that they could not access a host of websites which carried news or comment that Beijing deemed sensitive.
The Olympics led to an improvement in China's controls of the foreign media, and not all the advances have been rolled back, reports the BBC's Quentin Sommerville in Beijing.
But the country is expected to face a tough year ahead - the dramatic slowing of economic growth and rising unemployment are expected to fuel social unrest, he says.
It is also the 20th anniversary of the brutal suppression of the Tiananmen Square protests, and 50 years since China took direct control of Tibet.
More censorship and increased internal security are expected in 2009, our correspondent adds.