The only man convicted over the September 11 attacks has won the right to a retrial in a German appeal ruling that prosecutors say will hamper the pursuit of dangerous extremists.
Moroccan born Mounir El Motassadeq was sentenced to 15 years in jail in February 2003 for conspiring to murder nearly 3,000 people in the 2001 hijack attacks in the United States and for membership of a terrorist organisation, a German al Qaeda cell which included three of the September 11 suicide hijackers.
Supreme Court judge Klaus Tolksdorf said the state could not abandon principles of justice, however grave the crime.
"The fight against terrorism cannot be a wild, uncontrolled war," he said on Thursday.
The successful appeal was likely to be seen by the United States and German authorities as a major setback in what Washington calls its "war on terror". German Interior Minister Otto Schily described the decision as regrettable.
"It is quite clear that higher hurdles have been set for the prosecution of terrorism," state prosecutor Rolf Hannich said.
The U.S. embassy declined to comment.
Motassadeq has insisted he had no knowledge of the September 11 plot and did no more than help fellow Muslims living in a foreign country. His lawyers had argued that new evidence, which secured the acquittal of friend Abdelghani Mzoudi, on similar charges, also made Motassadeq's conviction unreliable.
They said they would demand Motassadeq, 29, be released from custody in Hamburg pending his new trial.
"It's a good verdict. It's a life-saving verdict," said Motassadeq's lawyer Josef Graessle-Muenscher.