The album has been controversial due to the cover image since its release more than three decades ago, and the Wikipedia article in question is "explanatory" in nature, describing and contextualizing "the controversy surrounding the image, in a neutral and educational fashion," Wikimedia devoted to the issue.

The chain of events got started on Dec. 4, when the IWF received a report from someone objecting to the Wikipedia article about the album that featured the cover image. Upon reviewing the user's complaint, the IWF, which works with law enforcement to block and remove child pornography online, decided to blacklist this Web page. ISPs that block pages based on the IWF's list then took action to shut off access to the offending Wikipedia article.

Once Wikipedia landed on the IWF blacklist, U.K. ISPs adopting the block began routing access to Wikipedia through a transparent proxy server. This made all U.K. Internet users indistinguishable by IP address, making them appear to Wikipedia as the same person. This in turn triggered a Wikipedia protective mechanism against abusive editing, leading the site to prevent these users from editing, according to Wikimedia.

Wikimedia estimates that there are "tens of thousands" of U.K. residents who regularly edit Wikipedia articles, amounting to about 25 percent of all edits to the English version of the encyclopedia.

The image in the Wikipedia article is hosted outside the U.K., an issue addressed by the IWF in its statement Tuesday. "Any further reported instances of this image which are hosted abroad, will not be added to the list. Any further reported instances of this image which are hosted in the U.K. will be assessed in line with IWF procedures."