The UK government is reportedly building a "whitelist" of legitimate websites to be exempted from its much-criticised web filter.

Internet service providers (ISPs) are currently required to ask new customers if they want to filter their internet for various content that might not be appropriate for children.

PM David Cameron said the filters were designed to stop young people "stumbling across hardcore legal pornography".

But the launch of those filters last year was widely criticised - both in principle, and for the fact that they seemed to block many legitimate websites along with material intended to be blocked.

In one notable case, the filter was found to block several sites offering sex education information specifically aimed at young people.

The filter will be extended from new customers to existing broadband subscribers later in 2014.

Ahead of that, the government is now trying to create a whitelist of sites that won't be blocked under any circumstances. The BBC reported that the idea came out of a working group, set up in the wake of the early controversy.

The group, chaired by David Miles for the UK Council for Child Internet Safety, said that "master list" was being drawn up with the help of charities and other groups.

A spokesman for the Internet Service Provides Association told the Beeb: "There's a growing realisation that filters are not perfect and will lead to some over-blocking,"

"There's a feeling that some sites sit in a grey area and more needs to be done for them."

However, for some the effort appears to be destined for failure, since the internet is generally able to thwart any attempt at categorisation or restriction: