Children's charities have expressed "serious concerns" many UK households still have access to images showing child sex abuse via their computers.
The government had asked all internet service providers (ISPs) to block illegal websites by the end of 2007. But firms providing 5% of broadband connections have still failed to act. One of them, Zen Internet, said in a statement: "We have not yet implemented the IWF's recommended system because we have concerns over its effectiveness."
It is understood other ISPs have cited the cost of blocking the illegal material as a reason not to participate in the scheme. But the NSPCC's Zoe Hilton said: "Allowing this loophole helps feed the appalling trade in images featuring real children being seriously sexually assaulted." The blocked websites come from a list supplied by the Internet Watch Foundation (IWF), but some smaller providers refuse to use the list.