Barack Obama's administration is being urged to lift a five-year ban on Tariq Ramadan, often described as Europe's foremost Islamic scholar, after a federal appeals court ruled that the government failed to justify denying him a US visa.
The ruling in New York on Friday was the latest legal blow to the controversial Patriot Act, adopted under the Bush administration following the attacks of September 11 2001, and a victory for civil rights groups that claim the anti-terror legislation has been used to stifle freedom of speech.
Swiss-born Mr Ramadan, an Oxford professor who has advised the UK government on tackling extremism, was denied a visa in 2004 to take up a teaching post at Notre Dame University in South Bend, Indiana. The government cited a provision of the Patriot Act that bars foreigners who endorse or espouse terrorism.
In a long-running legal battle, the authorities dropped the terrorism allegation but continued to bar Mr Ramadan on the grounds that he had donated money to a charity alleged to be connected to Hamas, the Palestinian militant group.
Mr Ramadan, 46, said that he was unaware of such a connection and the federal court ruled that the government failed to present evidence that he could answer.
The American Civil Liberties Union, which backed Mr Ramadan's case, said that it had written to the Obama administration asking it to grant him a visa immediately.
The case will now go back to a lower court where the government will have an opportunity to present evidence but ACLU officials thought such action unlikely.
"This is probably the end of the line," said Jameel Jaffer, head of the group's national security project. ACLU quoted Mr Ramadan as welcoming the court's decision.
"I am eager to engage once again with Americans in the kind of face-to-face discussions that are central to academic exchange and crucial to bridging cultural divides," he said.
The academic has publicly condemned terrorism and has urged young Muslims in the west to integrate.
The federal court said that the First Amendment freedom of speech rights of US organisations were threatened when foreign scholars, artists, politicians and others were excluded from the US.
The Patriot Act has suffered a number of setbacks in US courts, both from juries unwilling to convict on the basis of looser standards of evidence and from judges who insist on greater judicial oversight of its impact on civil rights and privacy.
Administration officials have nevertheless moved to renew some of its provisions in the face of demands by rights groups that the legislation should be scrapped.