The ISP in question is Demon Internet, which has made no formal announcement regarding the block of the Wayback Machine, one of the most popular Internet archives. Regular users of Alexa will know of the Wayback Machine's handy links in the left-hand column of every traffic stats page.
The blacklist originated with the Internet Watch Foundation, an organization that polices illegal online content. Despite the silence from Demon Internet, the IWF has confirmed its own involvement.
"The IWF can confirm it has taken action in relation to content on www.archive.org involving indecent images of children which contravenes UK law (Protection of Children Act 1978)," the IWF said in a statement. "The URL(s) in question were added to our URL list according to IWF procedures."
Demon Internet users first noticed the widespread ban when they tried to look up archived pages and got only generic "not found" pages that came from the IWF. Some Demon Internet users aren't experiencing any problems, while others have intermittent access to the Wayback Machine.
This isn't the first foul-up for the IWF. Last December, the IWF received complaints about an image on Wikipedia that depicted a naked prepubescent girl. The image in question was an album cover for the rock band The Scorpions.
In response to the complaints, the IWF instructed six British ISPs to reroute traffic through a small number of servers. The end result was that Wikipedia inadvertently stopped all British citizens from editing Wikipedia.