Some countries already operate systems of blocking, such as the UK's voluntary system run by the Internet Watch Foundation, a charity that is not a part of Government or the police, which maintains a list of sites which ISPs can choose to block.
The European Commission has proposed a new Directive on the sexual abuse and exploitation of children which proposes, amongst other measures, that countries force ISPs to block sites containing material depicting abuse.
"Member States shall take the necessary measures to obtain the removal of internet pages containing or disseminating child pornography," said a Commission statement summarising the proposed Directive.
"Member States will be obliged to ensure that access to websites containing child pornography can be blocked, as they are very difficult to take down at the source, especially if the site is outside the EU," it said. "The proposal will leave it to Member States to decide exactly how the blocking should be implemented but legal safeguards will always apply."
The move has been opposed by lobbying group European Digital Rights (EDRi), though. "As a measure which superficially sounds like a positive move, [forcing the blocking of sites] is also an attractive option politically, which creates the temptation to legislate based on impulse rather than on evidence, legality and effectiveness," said the group in a letter to European Commissioners.
"The proposal is flawed on the basis of law, flawed on the basis of possible effectiveness, flawed on the basis of unintended consequences for the fight against online child abuse and flawed on the basis of inevitable damage for freedom of communication and privacy in the online world," it said.
The letter was to Home Affairs Commissioner Cecilia Malmström; Justice Commissioner Viviane Reding; and Digital Agenda Commissioner Neelie Kroes.
EDRi said that the Commission's policies were based on misunderstandings of the problem.
"As repeatedly and consistently shown by figures produced by EU hotlines, the websites that are targeted by blocking measures are not in some distant 'rogue states' that the EU has no influence over – they are hosted on the territories of our major trading partners," it said. "What is needed is comprehensive international law enforcement cooperation – the last thing that is needed is a policy which does nothing to address the actual problem but reduces the political pressure for effective action."
The group warned that blocking is technically ineffective and possibly forbidden by the European Convention on Human Rights. It says that governments could use the blocking in the future for political or economic reasons if it is allowed to be used in the battle against distributors of material depicting child exploitation.
The Commission said that its existing Framework Decision on the issue was inadequate, and that new rules were needed.
"[The Framework Decision] approximates legislation only on a limited number of offences, does not address new forms of sexual abuse and exploitation using information technology, does not remove obstacles to prosecuting offences outside national territory, does not meet all the specific needs of child victims, and does not contain adequate measures to prevent offences. This calls for a substantive improvement of EU rules," said the Commission in a statement.