San Francisco - Online encyclopaedia Wikipedia will shut down its English language site on Wednesday for 24 hours to protest at proposed anti-piracy legislation in the US.
The unprecedented move comes in reaction to two bills making their way through the US houses of Congress, the Stop Online Piracy Act (SOPA) and the Protect IP Act (PIPA).
Critics of the legislation maintain that they would limit free speech and give content owners draconian powers to shut down websites implicated in piracy, even if they merely provide links to copyrighted content.
"This is an extraordinary action for our community to take," Wikipedia founder Jimmy Wales said on Tuesday.
"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."
With over 25 million unique visitors per day Wikipedia's US site is the largest web property taking part in the online blackout. Other participants include news site Reddit, the left-wing advocacy group MoveOn, Mozilla, the maker of Firefox, and the popular blog Boing Boing.
The proposed legislation is strongly supported by US movie and music industries who say web sites should do more to curb the distribution of pirated material.
Those opposing the proposed legislation include Google and Twitter though both have declined to join the blackout. "Closing a global business in reaction to single-issue national politics is foolish," Twitter CEO Dick Costolo said.
Google said it plans to place a link on its home page to highlight its opposition to the anti-piracy measures.
The White House has indicated it opposes the legislation.
"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 cybersecurity risk, or undermines the dynamic, innovative global internet," it said in a blog posting last week.