Wikipedia allows readers to edit the content of its pages
A decision by a number of UK internet providers to block a Wikipedia page showing an image of a naked girl has angered users of the popular site.
The blocked page of the online encyclopaedia shows an album cover of German heavy metal band Scorpions, released in 1976.
Internet providers acted after online watchdog the Internet Watch Foundation warned them its picture may be illegal.
The IWF said it was a "potentially illegal child sexual abuse image".
Some volunteers who run Wikipedia said it was not for the foundation to censor one of the web's most popular sites.
They also argued that the image was available in a number of books and had never been ruled illegal.
But the IWF, which warns internet providers about possible images that could be linked to child abuse, said it had consulted the police before making its decision.
The foundation's list of proscribed sites is widely used by British internet service providers to filter out images showing child abuse and other illegal content.
As a result, the addition of the Scorpions Wikipedia page has made it inaccessible to the majority of British internet users.
The IWF, which lists the BBC, News International and internet companies AOL (UK) and Ask among its members, said as many as 95% of British users would now be unable to access the page.
Wikipedia volunteer David Gerard said he and fellow users were angry that as well as the photo, the text on the page had been blocked.
"Blocking text is a whole new thing - it's the first time they've done this on such a visible site," he said.
Mr Gerard also told BBC Radio 4's Today programme the album cover was still available as part of the band's box set and could be viewed on retail websites.
"When we asked the Internet Watch Foundation why they blocked Wikipedia and not Amazon, apparently the decision was 'pragmatic', which we think means that Amazon had money and would sue them, whereas we're an educational charity."
Susan Robertson, of the IWF, said the image could potentially contravene the Protection of Children Act 1978.
"We only act on the reports we receive, and as I understand it, the only report we received regarding this content, as of Friday, was the content on Wikipedia," she said.
Ms Robertson also said the IWF needed to "take a view" on the images available on Amazon with its "analyst team and police partners".
Jay Walsh, a spokesman for the Wikimedia Foundation, which manages the encyclopaedia, said the removal of the page also appeared to have stopped thousands of UK users from editing articles on Wikipedia, which allows readers to self-edit its pages.
"It appears that there's a large number of editors - I can't say all - who appear to have access issues," he said.
The IWF spokeswoman said a reader had brought the image to the foundation's attention last week and it had contacted the police before adding the page to their list.
Wikipedia is one of the world's most popular websites. It is a multi-lingual online database written, edited and funded largely by its users. It has 2.6m articles in English alone.