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.
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.
