At issue is a statute that was the first to criminalise offering "material support" to foreign terror groups. That law was the precursor to the controversial 2001 Patriot Act which expanded the government's intelligence-gathering powers and increased penalties for activities classified as terrorist.

In their decision, the 9th Circuit Court of Appeals upheld an earlier preliminary ruling that prohibited the provision of "personnel" and "training" to groups designated by the United States as "terrorist organisations."

The court also ruled that before applying the law the government must prove beyond a reasonable doubt that a donor to a group branded as a "foreign terrorist organisation" knew of its unlawful activities.

"The prohibition on providing "training" and "personnel" is impermissibly overbroad and thus void for vagueness under the First and Fifth Amendments," the court ruled.

That "personnel" provision was used to indict "American Taliban" John Walker Lindh and six people in Buffalo, New York knows as the "Lackawanna Six," said David Cole, a lawyer for the Center for Constitutional Rights which brought the case.

He added the ruling that covers the Western states in the 9th Circuit could make it tougher for the Bush administration to prosecute individuals charged with aiding designated terrorist groups.