The Internet Watch Foundation has lifted its ban on a Wikipedia page and image of a record album cover showing a young nude girl, in an unprecedented move.

The IWF also acknowledged has its initial ban had the reverse effect that it wished — and spread the image further around the net instead of removing it.

The initial move last Friday by the IWF, which acts as a watchdog — and in effect censor — for internet content visible in the UK meant that some people could not see any pages on Wikipedia at all, while others were unable to edit pages on the user-generated encyclopedia.

The offending picture, from the 1976 album cover Virgin Killer by the German rock band Scorpions, showed a nude girl with a crack in the camera lens obscuring her genitals. The band has said that the choice of picture was made by their record company, and it was later changed.

The IWF banned it after the web page location was sent to it last Thursday. But a protest from the Wikimedia organisation on Monday led the IWF into an unprecedented reconsideration of its earlier ban.

Normally the IWF bans about 10,000 web pages from around the world every year, adding them to a blacklist whose contents are kept secret — but implemented — by all the biggest internet service providers and mobile operators.

After more than a day of consideration, the IWF has issued a press release in which it says that it has lifted the ban, after considering the "context" of the image — something which it had previously said would not affect the legitimacy of an image.

"The procedure is now complete and has confirmed that the image in question is potentially in breach of the Protection of Children Act 1978," the statement says. "However, the IWF Board has today (9 December 2008) considered these findings and the contextual issues involved in this specific case and, 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."

That is a surprising move, because it opens up the possibility that any site which finds itself blocked could claim that its content is, contextually, artistic — and so get a ban reversed. It also throws into question the application by the IWF of its banning system, which is carried out by a four-strong team of analysts who work with the police's Child Exploitation and Online Protection (CEOP) unit.

Copies of the image hosted abroad — such as on the Amazon US shopping site, which until yesterday did have the cover — will not be subject to a ban, the IWF said. But it added that "any further reported instances of this image which are hosted in the UK will be assessed in line with IWF procedures".

That means that an image could be considered pornographic in the UK but not the US — further confusing the role of the IWF. The IWF also acknowledged that its efforts had had the reverse effect that it had intended.

"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."