A US federal appeals court has rejected rules intended to prevent internet service providers (ISPs) from prioritising certain types of content.
Net neutrality is the principle that ISPs should not block web traffic for customers who pay less to give faster speeds to those who pay more.
The US Federal Communication Commission (FCC) adopted the rules in 2010.
They were challenged by ISP Verizon, which claimed the FCC was overstepping its legal authority. The court agreed.
Supporters of net neutrality said the ruling was a major threat to how people use the internet.
The rules were designed to ensure that small or start-up organisations had as much chance of reaching an online audience as a large, established company.
But broadband providers argue that some traffic-heavy sites - for example, YouTube or Netflix - put a strain on their infrastructure.
They say they should be able to charge such content providers so that users who pay more can get faster access to those sites than other customers.
As a consequence, companies who did not pay would find that access to their services could be slower for customers.
Verizon had said in September 2013 that if it were not for net neutrality rules they would be looking at different pricing models.
In a statement released after the ruling Verizon said that the court's decision would not affect customers' ability to access and use the internet as they do now.
"The court's decision will allow more room for innovation, and consumers will have more choices to determine for themselves how they access and experience the internet," it said.
The boss of BitTorrent - a system for sharing large files using peer-to-peer technology - warned that the court's decision would be a major threat to innovation, free speech and "the internet as we know it."
"For the ISPs, it's a momentous decision. This ruling will consolidate their powerful role as arbiters of culture and speech.
"And we can expect this consolidation to escalate now that big content and big networks have been given an unambiguous green light.
"For everyone else, permission to play - the ability to even compete - will come at a cost," said chief executive Eric Klinker
The FCC said it would consider appealing.
Chairman Tom Wheeler said: "We will consider all available options, including those for appeal, to ensure that these networks on which the internet depends continue to provide a free and open platform for innovation and expressions."