The online watchdog, the Internet Watch Foundation (IWF), has withdrawn its objection to a Wikipedia page that contained an image of a naked girl.
The page of the online encyclopaedia shows an album cover of German heavy metal band Scorpions, released in 1976.
A number of internet providers blocked the page after IWF said it could be "potential illegal child sexual abuse."
The IWF now says that given the age and availability of the image, it was no longer on its list of proscribed sites.
IWF's overriding objective is to minimise the availability of indecent images of children on the internet, however, on this occasion our efforts have had the opposite effect
Internet Watch Foundation
They also argued that the image was available in a number of books and had never been ruled illegal.
In a statement on its website, the IWF said that the image could still potentially breach the Protection of Children Act 1978, but "in light of the length of time the image has existed and its wide availability, the decision has been taken to remove this webpage from our list."
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.
The IWF admitted that its attempts to prevent people seeing the image had been counter productive."IWF's overriding objective is to minimise the availability of indecent images of children on the internet, however, on this occasion our efforts have had the opposite effect. We regret the unintended consequences for Wikipedia and its users."