BT and TalkTalk have launched a legal challenge over the introduction of the Digital Economy Act.
The companies have filed papers with the High Court asking for a judicial review into the act, which was controversially passed earlier this year.
Both firms believe the act did not receive enough scrutiny before it was "rushed" into law towards the end of Labour's time in power.
BT and TalkTalk claimed citizens could be "harmed" by the legislation and the companies are asking for clarity on the act before they are asked to implement it along with other firms.
Concerns were also raised over the act's compatibility with European rules designed to ensure national laws are "proportionate", maintain a single market and protect users' privacy, while restricting the part ISPs play in policing the internet.
"Throughout the passage of the Digital Economy Bill we expressed concerns about rushing through legislation on matters as important as web blocking and intereference with internet access. We felt that insufficient time had been allowed for proper debate in general, including significant changes made to the text of the bill very late in the day," a BT spokesman told IT PRO.
"We believe the previous government did not properly inform other European Union member states of the new law, which we believe breaches EU legislation on privacy and how internet companies process customer data," he added.
Charles Dunstone, chairman of the TalkTalk Group, claimed the act will cost the UK hundreds of millions, adding that many believe the measures "are unfair, unwarranted and won't work".
"So it's no surprise that in Nick Clegg's call for laws to repeal, this act is top of the public's wish list'. Innocent broadband customers will suffer and citizens will have their privacy invaded. We think the previous Government's rushed approach resulted in flawed legislation," Dunstone said.
The act was passed in April this year after less than two hours of debate at the committee stage and third reading.
