Wikipedia is to black out its English-language website for 24 hours in protest over US legislators' Stop Online Piracy Act (Sopa), criticised for having the potential to censor the internet, despite key parts of the act being watered down by the White House.
The White House announced it will not approve crucial parts of the act while major players such as Sony, Nintendo, Electronic Arts and Anonymous, have lined up against it.
Wikipediasaid in a statement: "The Wikipedia community has chosen to black out the English version of Wikipedia for 24 hours in protest against proposed legislation in the United States - the Stop Online Piracy Act in the US House of Representatives and Protect Intellectual Property Act (Pipa) in the US Senate.
The shutdown was planned for 18 January.
"If passed, this legislation will harm the free and open internet and bring about new tools for censorship of international websites inside the United States," Wikipedia said of Sopa and Pipa.
Despite the vote on Sopa being delayed indefinitely becuase of political pressure, social news site Reddit will join Wikipedia in the blackout to protest against Pipa.
The website planned to be unavailable from 5am GMT for 24 hours and will replace its vast database of nearly four million articles with an open letter encouraging visitors to contact Congress in protest at the planned legislation.
Wikipedia co-founder Jimmy Wales said on Twitter that he hoped the issue would "melt switchboards" in Washington.
"We have no indication that Sopa is fully off the table," he said. "We need to send Washington a BIG message."
Wikipedia is the sixth most-visited website on the internet and attracts 25 million visitors a day.
The White House said: "While we believe that online piracy by foreign websites is a serious problem that requires a serious legislative response, we will not support legislation that reduces freedom of expression, increases cyber-security risk, or undermines the dynamic, innovative global internet."
In a joint letter to members of the Senate in November, Facebook, Google and Twitter said: "The bills as drafted would expose law-abiding US internet and technology companies to new uncertain liabilities, private rights of action and technology mandates that would require monitoring of websites."
If Sopa were passed as it stood, minor offences such as uploading copyrighted material to YouTube or posting a link on Facebook could lead to a prison sentence.
Wales said: "This is an extraordinary action for our community to take and while we regret having to prevent the world from having access to Wikipedia for even a second, we simply cannot ignore the fact that Sopa and Pipa endanger free speech both in the United States and abroad and set a frightening precedent of internet censorship for the world."