The Internet Watch Foundation (IWF) is reviewing its decision to list as "child pornography" the image on one version of the album "Virgin Killer" by the rock band The Scorpions hosted on Wikipedia – and might yet add Amazon US to its list of "blocked" sites for hosting the picture.

That could be disastrous for Amazon if it prevents people in the UK accessing its pages on the week that is expected to be the busiest online before Christmas.

The initial decision to block the image, taken on Friday, prevented UK contributors from editing the site, and blocked some people from seeing the site at all (although they were still able to view it through Google's cache).

That is because the IWF's system adds both the URL of an image and of a page containing the image to its "blacklist" of pages to be blocked. "Illegal sites often hide images in pages," said Sarah Robertson, director of communications for the IWF.

The decision to ban the page, which was taken after consultation with the UK's Child Exploitation and Online Protection (CEOP) agency, is now being reviewed, Robertson said. "The assessment was done in partnership with law enforcement."

The Scorpions image was deemed to be "1 on a scale of 1 to 5, where 1 is the least offensive", said Robertson. The image was judged to be "erotic posing with no sexual activity". It depicts a young naked girl with her genitals obscured by a crack in the camera lens.

Robertson declined to say whether Amazon would be the next to be blocked. She confirmed that the Amazon page containing the offending cover was referred to the IWF today, but that no decision would be taken while the review of the original decision was in progress.

She could not say how long the review of the original decision will take. Typically it takes about 24 hours for a URL to go from initial referral to being added to the blacklist.

The decision has put the IWF's methods and systems under the media spotlight. Normally the IWF, which is paid for by the EU and through a levy on the internet industry, works quietly away in its Cambridge offices. A team of four police-trained "analysts" plough through 35,000 URLs sent to them each year that are under suspicion of being obscene.

That works out to an average of 700 per week, or 140 per working day, or 35 per working day per analyst - giving each an average workload for a seven-hour day of 5 URLs per hour. Typically about one-third of the URLs are deemed illegal.

If an image or text page contains obscene content and is hosted in the UK, the relevant ISP is contacted and the content removed. But if it is hosted abroad, it is added instead to a "blacklist" to which access is prevented by BT's "CleanFeed" technology. Any attempt to access that page returns a "Page Not Found" response.

"We were notified of this [Wikipedia] URL last Thursday and added it to our blacklist on Friday," Robertson said.

The image under consideration was previously considered by the FBI in the US.

In an interview with a Portuguese magazine, the leader of the Scorpions said that the decision to use the image had not been the band's: