Two of the UK's biggest Internet service providers today begin their attempt to overturn the UK's Digital Economy Act (DEA) at the High Court in London.
BT and TalkTalk are challenging the legality of the Act on the basis it is in breach of European privacy and human rights legislation.
The judicial review process will delay the implementation of the Act by two years. If the controversial legislation eventually comes into force, it could see ISPs acting as policemen to their own customers, and footing a sizeable bill for providing the names and addresses of alleged copyright infringers.
The two ISPs together boast 8.4 million UK subscribers, and are already believed to have spent £1 million in legal fees on challenging the Act.
Critics claim that the DEA suffered from a lack of scrutiny, after being railroaded through parliament by former business secretary Peter Mandelson during the dying days of the last Labour government
Prior to the 2010 general election, Conservative shadow culture secretary minister Jeremy Hunt claimed the Tories would repeal "flawed" clauses in the legislation if elected. In spite of that pledge, no government review has been announced - leading the ISPs to mount their own legal challenge.
Not all ISPs have spoken out against the Act's anti-piracy measures. A number of broadband providers are in talks with copyright holders to plug the two-year gap in implementation with a voluntary 'blacklist' of around 100 suspect sites that includes notorious BitTorrent tracker The Pirate Bay.
Industry bodies, including actors' union Equity and UK record industry body the BPI, have also thrown their weight behind the Act, claiming that its draconian anti-piracy provisions are necessary to safeguard their industries.
A central pillar of the Act's enforcement - identifying individual users via their IP addresses - has also been called into question by the judge in a recent High Court case.
Dismissing the arguments of cowboy legal outfit ACS Law - the firm that knowingly demanded sums of £500 or more from users it accused of copyright infringement - Judge Colin Birss QC said that problems remained with proving the link between an IP address and an individual subscriber.
The DEA judicial review hearing is expected to last six weeks. Live updates from court are available on Twitter today from @Morus1516.