Wikipedia is the sixth-most visited website in the world, according to Alexa. Opponents of SOPA argue they impose unfair responsibilities on websites such as Wikipedia to check that no material they host infringes copyright.

Wikipedia’s co-founder Jimmy Wales called the decision to shut the website an “extraordinary” action as the proposed laws “endanger free speech both in the United States and abroad, and set a frightening precedent of Internet censorship for the world.”

Under current laws, if websites remove pirated content when they are notified by the holder of copyright they are not liable for damages.

The proposed laws also make it easier for American copyright holders to cut off access to foreign websites, which are hosting pirated, illegal or unlicensed copies of TV programs, movies and music.

The proposed legislation has been supported by am aggressive lobbying campaign by major media owners, including Rupert Murdoch, and opposed by the Silicon Valley players such as Facebook and Google.

However, Twitter’s Dick Costolo called Wikipedia’s plans to pull the plug on its website “foolish” and “silly”, via Twitter (funnily enough).

“That’s just silly. Closing a global business in reaction to single-issue national politics is foolish,” Costolo replied to a technology journalist’s question about the issue.

The English language version of the website will be down from 5AM GMT on Wednesday until 5AM GMT on Thursday.