The ISP Association has said it is outraged at an amendment to the Digital Economy Bill that would let copyright holders force ISPs to block websites accused of unlawfully giving access to copyrighted material.
In a statement on Thursday, ISPA said Amendment 120a, passed in the House of Lords on Tuesday, was "hastily constructed and rushed through at report stage without due consideration of the implications or consultation with the interested parties that would be affected".
The amendment, which permits copyright holders to apply for high court injunctions to shut down suspected infringing sites, was successfully proposed by Liberal Democrat peer Lord Clement-Jones and Conservative peer Lord Howard of Rising.
The measure is partly intended to replace Clause 17, a section of the bill that would have given the business secretary the right to amend copyright legislation to meet changes in technology used to break copyright.
"ISPA has been supportive of peers' excellent scrutiny of the bill to date," ISPA general secretary Nicholas Lansman said in the statement. "However, in this instance, our members are extremely concerned that the full implications of the amendment have not been understood and that the reasoning behind the amendment is wholly misguided. We would therefore urge the Conservatives and Liberal Democrats to urgently reconsider their position."
The ISP trade body said the amendment is biased in favour of rights owners. It also noted that the lords behind the amendment have drawn parallels between blocking copyright-infringing sites and blocking child pornography sites.
"The suggestion that a framework developed to fight against the distribution of criminal images of child sexual abuse is appropriate to tackle allegations of civil copyright infringement is incomprehensible," ISPA said.
Speaking to ZDNet UK on Thursday, Clement-Jones said he had used the analogy "as an illustration to demonstrate that site blocking already exists".
"It is perfectly legitimate to do that," Clement-Jones said. "This is not a novel concept. There are sites that are identified as being blocked in various fields. I am not saying child pornography is equivalent to copyright infringement."
Clement-Jones denied there was a distinction to be made between blocking for criminal and civil offences. "If you're infringing somebody's copyright on the web, it's something that should not be taking place," he said.
ISPA pointed out that network-level blocking was "ineffective", as people can get around these restrictions by using various technological means such as proxies and encryption.
Clement-Jones said he did not think such workarounds would be "a universal way of behaving". He also said the blocking of child pornography websites had not led to a growth in the use of encryption to avoid such restrictions.
Amendment 120a also drew the ire of security researcher Richard Clayton of the University of Cambridge.
"This is a completely nonsensical, unworkable proposal," Clayton said. "It appears to assume that ISPs can just block sites, which is only workable if you block the whole IP address."
Clayton added that the amendment's proposed framework for takedown requests from rights holders could lead to a "huge chilling effect" on the flow of information on the web.
"The Daily Telegraph set up a site which showed MPs' expenses, which are Crown Copyright," Clayton said. "If this law was in effect, any MP could get that site taken down."
However, Clement-Jones said he did not believe his amendment, in its current form, raised "any possibility of censorship".
ZDNet UK's Tom Espiner contributed to this report.