Despite the UK government asking ISPs to ban access to websites containing illegal child abuse images by the end of 2007, broadband ISPs still allow more than 700,000 households to see these images, according to the Children's Charities' Coalition on Internet Safety.

The group, which is backed by the NSPCC, Barnardo's and The Children's Society, said it has "serious concern" over the "seriously damaging" situation.

In 2006, the government asked all UK ISPs to block access to any websites on the Internet Watch Foundation's list of illegal sites. However by June 2008, the government revealed that 5 percent of the UK's ISPs still had not implemented the move.

"Allowing this loophole helps to feed the appalling trade in images which feature real children being seriously sexually assaulted," said Zoe Hilton, policy adviser for the NSPCC.

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"Self-regulation on this issue is obviously failing - and in a seriously damaging way for children, we now need decisive action from the government to ensure the ISPs that are still refusing to block this foul material are forced to fall into line."

Zen Internet, one of the ISPs that does not block sites on the IWF's list, said: "We have not yet implemented the IWF's recommended system because we have concerns over its effectiveness".

Meanwhile it's thought that other ISPs have claimed it's too expensive to block the illegal material. The government said it was "looking at ways to progress the final 5 percent".

See also: ISPs block access to 'illegal' Wikipedia page