The US supreme court on Tuesday barred a law that sought to shield minors from Internet pornography on the ground that it interfered with the constitutional right of free speech.

The court, however, decided to embrace an Internet filtering software, which was preferred over such laws, as it is less restrictive and probably more effective at keeping children from viewing Internet smut.

It was the third time in seven years that a Congressional effort to curb online obscenity failed the test in court.

"Content-based prohibitions, enforced by severe criminal penalties, have the constant potential to be a repressive force in the lives and thoughts of a free people," Justice Anthony M Kennedy wrote for the court majority.