The US broadcast regulator has been told by appeal judges it has "crossed the line" with an anti-piracy tag which stops programmes being copied.
The courts said the FCC should not decide how devices work
The "broadcast flag" is a small bit of data attached to US digital broadcasts. It tells devices that receive digital signals the level of copy protection.
From 1 July, any device that cannot read the flag will be illegal to make.
But the panel of appeal judges said the Federal Communications Commission (FCC) should not dictate how devices work.
"You crossed the line," Judge Harry Edwards told a FCC lawyer during arguments before a three-judge panel of the US Court of Appeals for the DC Circuit.
"Selling televisions is not what the FCC is in the business of."
Supporters of the flag say it will help combat piracy by preventing people copying digital TV programmes and distributing them over the net.
The mandate was brought in after pressure from the US entertainment industry which is keen to protect high-quality broadcasts.
TV piracy is a growing concern for broadcasters, particuarly as next-generation high-definition devices and programmes become more popular.
Downloads of TV programmes have increased by 150% in the last year, according to a recent report, with a typical episode of the popular TV series 24 downloaded by about 100,000 people globally.
But critics say the flag also stops people from making legitimate personal copies of shows, or copies for educational or teaching purposes.
They are also concerned that the rule would mean the FCC has the right to say how TVs, computers, and other devices capable of receiving digital signals, are built and used.
It could also mean, they say, next-generation TVs and other receiver technologies are more expensive.
The entertainment industry is concerned about technology that facilitates piracy
The judges said that the rule could set a precedent for other wide-ranging FCC regulations on future innovations.
The flag is one of the technologies that was identified by the net rights group, the Electronic Frontier Foundation (EFF), as one of the biggest threats to the survival of certain devices, like high-definition PC tuner cards.
EFF lawyer Wendy Seltzer told the BBC News website that the judges comments on the FCC's authority were welcome.
"The judges' questioning of the FCC's authority was right on target," she said.
"The broadcast flag bears about as much relation to the FCC's mandate as dishwashers.
"We're encouraged by the reports we've heard. Of course we're still keeping the HDTV tuner cards on our 'endangered' list."
The Endangered Gizmos list is designed to draw attention to the laws and regulations advocated by the entertainment industry that may threaten the future development of certain gadgets and technologies.
The appeals panel now has to decide whether consumer groups which are criticising the rule should have the right to contest the FCC's requirements.
The case could be thrown out of the appeals court if it decides they do not have the right to lawfully challenge the FCC decision. A decision by the court is expected within months.
The FCC ruling only applies to receiver equipment compatible with the US digital terrestrial TV system, a spokeswoman from the UK broadcast watchdog Ofcom told the BBC News website.
The UK digital terrestrial platform, Freeview, and other receivers do not employ a copy protection technology.
Individual European Union member states are not allowed to mandate receiver requirements and any copy protection system would need to be agreed at a European level.