A senior US judge has overturned a law designed to protect children from viewing harmful material on the web.

In a landmark decision, senior US District Judge Lowell Reed Jr struck down as "unconstitutional" the 1998 US law, part of the Child Online Protection Act, that makes it a crime for commercial website operators to let children access " harmful" material.

The judge said that the definition of the law was "too broad", and suggested that technology has advanced sufficiently to mean that today's software filters do the job of protecting children much better than in 1998.

He said that the existing legislation was not flexible enough to take into account the latest threats such as online grooming of minors on social networking sites.

David Milman, chief executive at independent computer repair company Rescuecom, said: "For many parents, their children know as much if not more about their computers than they do.

"However, this decision puts the burden squarely on parents to take action to protect their children."

The company warned that parents need to take steps to protect their children, including using content filtering software such as Net Nanny or CyberPatrol.

Parets should also locate computers in the family room, living room or otherwise open area in the home, limit the amount of time children spend online, educate children to the dangers of online threats, and make sure software is up to date.

"Now more than ever it is vital to make sure our children are protected and that the computers in our homes have the proper filters and warning systems against harmful content. This protection is easy to put in place," Milman said.