Attempts to guarantee "net neutrality" - equal access to the internet for all users - suffered a setback in the US, when an appeals court backed Comcast in a dispute over whether the cable firm could slow down the internet for customers who used file-sharing software.

Comcast had argued that federal regulators had no right to interfere with how it managed traffic on its network, even though it had stopped its actions against heavy bandwidth users following an outcry.

The Federal Communications Commission censured Comcast in 2008 in a ruling that was hailed as an important step towards ensuring net neutrality.

Internet providers have been lobbying that they should be allowed to charge more for better network access, but campaigners complain that this could confine bloggers and start-up firms to an internet "slow lane", while big corporations are favoured.

The court "dropped a bomb on us", said Ben Scott, policy director of the advocacy group Free Press that challenged Comcast and sided with the FCC, in an interview with Bloomberg. "Comcast is now permitted to block websites with impunity."

Comcast argued that it had imposed the restrictions to make sure that heavy internet users didn't cause a slower service for other customers.

The FCC, under a new chairman appointed by the Obama administration, Julius Genachowski, has promised to safeguard net neutrality and proposed a set of rules that are currently the subject of public consultation.

The issue has moved up the agenda as video is increasingly available over the web, meaning that mushrooming internet traffic has put pressure network capacity.

But the ruling by the US Court of Appeals complicates the FCC's efforts to impose those new rules, which are almost certain now to face legal challenge.

Judges said the commission failed to show it had the necessary authority to impose such restrictions on network management.

"It relies principally on several congressional statements of policy, but? statements of policy, by themselves, do not create 'statutorily mandated responsibilities,'" the three-judge panel said.

The FCC could appeal to the Supreme Court or seek help from Congress where lawmakers could rewrite the laws to provide the agency more explicit authority. The agency could also try to rewrite its own rules to try to address the issue.