The world's second largest web browser has been attacked by GCHQ over plans that would create a 'Yellow Pages' of child porn.

The spy agency has described demands by Mozilla as “completely unacceptable” after the company said it would only block websites on its popular Firefox browser if the complete list of those sites was made public.

That would mean the UK’s child abuse watchdog, the Internet Watch Foundation (IWF), which issues requests to prevent access to indecent sites, would have to publish its “block list”.

Mozilla responded saying that the concerns were "misplaced" and that the current system of blocking websites did "more harm than good".

Speaking at a recent conference, a spokesman the National Cyber Security Centre, an arm of GCHQ, said: "That is something that we feel is completely unacceptable when it comes to abusive content online.

“It may make sense in the context of other countries’ censorship, but it is unacceptable for child abuse websites to be listed online."

Most website blocking in the UK is currently carried out by British-based internet service providers (ISPs) such as BT and Sky, who connect users with the websites they visit.

However, tech giants such as Google and Mozilla are planning to change their browsers so they take users directly to websites.