BT and TalkTalk are seeking a judicial review of the controversial Digital Economy Act, BBC News has learned.
The two internet service providers want the High Court to clarify the legality of the act before it is implemented.
The act was "rushed through" parliament before the general election, they say.
Both think it had "insufficient scrutiny" and question whether its proposals to curb illegal file-sharing harm "basic rights and freedoms".
The act became law shortly before parliament was dissolved in the so-called wash-up period.
It meant it was subject to a shorter debate than other acts. MPs from all parties, including deputy prime minister Nick Clegg, protested at the time that the complex bill should have been debated for longer.
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"It means we could have huge swathes of customers moving to smaller ISPs to avoid detection" - Andrew Heaney TalkTalk
Among its most controversial measures were proposals to disconnect persistent illegal file-sharers from the web and give copyright holders the power to block access to websites hosting illegal content.
Regulator Ofcom, charged with drawing up detailed plans of how the legislation will work, has recently said that plans to remove peoples' internet connections would not come into force until at least 2011.
A caveat added to the act at the last minute stipulates that new legislation would be required before such measures are implemented.
In May Ofcom drew up the policy to deal with illegal file-sharers. It requires ISPs to send warning letters to customers who illegally download films, music and TV programs.
Persistent pirates will be put on a blacklist and their details can be passed to relevant copyright owners to pursue the case through the courts should they wish to.
The code of practice currently only applies to larger ISPs with more than 400,000 subscribers.
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" We believe measures are consistent with EU legislation and that there are enough safeguards in place to protect the rights of consumers and ISPs " - Department of Business, Innovation and Skills
This puts BT, TalkTalk and the other large ISPs at a business disadvantage, said Andrew Heaney, executive director of TalkTalk.
"It means we could have huge swathes of customers moving to smaller ISPs to avoid detection."
In particular TalkTalk and BT are seeking clarity as to whether the act conflicts with EU legislation.
It could conflict with Europe's e-commerce directive which states that ISPs are "mere conduits" of content and should not be held responsible for the traffic on their networks.
It may also be in contravention of the privacy and electronic communications directive, said Mr Heaney.
The BPI, which represents the UK's recorded music industry, has lobbied hard for the Digital Economy Act and has taken legal action against file-sharers in the past.
Right to repeal
"It is outrageous that they are coming begging at our door but are not helping themselves," said Mr Heaney.
Critics believe the music industry is seeking to protect its old business models with legislation, rather than finding new ways to distribute music online.
The current government has the right to repeal any previous legislation and, during the election campaign, deputy prime minister Nick Clegg said that the Digital Economy Act "badly needs to be repealed".
But the coalition government told the BBC it had no plans to change it.
"The Digital Economy Act sets out to protect our creative economy from the continued threat of online copyright infringement, which industry estimates costs the creative industries, including creators, £400m per year," read a statement from the Department of Business, Innovation and Skills.
"We believe measures are consistent with EU legislation and that there are enough safeguards in place to protect the rights of consumers and ISPs and will continue to work on implementing them."
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