Internet service companies could be obliged by law to provide an automatic block to stop children being able to view hard core pornography online.
The Government is expected to discuss proposals with the industry that would force ISPs to give parents the right to ban all pornographic material from their home internet accounts.
David Cameron will say the current law needs to be examined to ensure families can use the internet in safety. However, critics have warned that such a bar would be impractical, expensive and draconian. They suggested the consultation was more about appeasing right wing newspapers that have been campaigning for such a filter and deflecting attention from the party's expected poor performance.
Nick Pickles, director of Big Brother Watch, said: "Mass blocking of websites was rejected in the US for breaking internet security and in Holland because it doesn't work.
"The focus should be on how parents can have the tools to protect their children without the Government deciding what we can all see online.
"As recognised by Ofcom, it is trivial to get around an opt-in blocking system, so it would be totally wrong for this to be used as an excuse to spy on all our internet activity."
Nicholas Lansman, ISPA secretary general, said: "Forcing ISPs to filter adult content at the network level, which users would then have to opt out of, is neither the most effective nor most appropriate way to prevent access to inappropriate material online.
"It is easy to circumvent, reduces the degree of active interest and parental mediation and has clear implications for freedom of speech. Instead parents should choose how they restrict access to content, be it on the device or network level with the tools provided."
The main phone companies signed a voluntary pledge in 2004 to filter certain adult content on most mobiles – meaning many users are being forced to contact their provider to lift the block. But there is no similar 'opt-in' obligation on internet service providers. Pressure was raised on the Government to act last month after a group of MPs issued a report which claimed that six out of 10 children download adult material because their parents have not installed filters. The use of protective filters in homes has fallen from 49 per cent to 39 per cent in the last three years.
The report – following an investigation chaired by the Tory MP Clare Perry – concluded that parents were often outsmarted by their web-savvy children and felt unconfident in updating and downloading content filters.