An amendment proposed to the Digital Economy Bill would force internet service providers (ISPs) to block pornographic websites if they fail to verify the age of a visitor.

The bill, which is now in the final stage of consideration in Parliament, originally set out to impose age restrictions on all pornographic websites, forcing users to state how old they were before accessing material.

The bill states that "a person must not make pornographic material available on the internet on a commercial basis to persons in the UK, except in a way that secures that, at any given time, the material is not normally accessible by persons under the age of 18."

Now a group of MPs have put forward a change that would block sites that failed to assess the age of their visitors, determined by a government-appointed 'age verification regulator'.

The government believes the new powers will help protect children from accessing inappropriate material on the internet.

However, Open Rights Group (ORG), a digital free speech organisation, argued the bill is an infringement on human rights, and a complete block on content should only be reserved for those sites showing illegal material.

"Perhaps these MPs have realised that plans to make all adult websites apply age verification are unworkable as foreign porn sites may simply not comply," said Jim Killock, executive director of ORG.