Spain’s supreme court today quashed the conviction of a Syrian-born businessman for conspiracy in the September 2001 terror attacks on New York and Washington.

Although Imad Eddin Barakat Yarkas, known as Abu Dahdah, still has to serve a 12-year sentence for membership of the al-Qaeda network, the Spanish court's decision leaves Zacarias Moussaoui as the only man anywhere in the world jailed for the 9/11 attacks.

It also calls into question the extradition from Britain to Spain of the Moroccan Farid Hilali, who Spanish prosecutors say was a co-conspirator in the US attacks.

Yarkas, thought to have been the head of the al-Qaeda cell in Madrid, was one of 18 al-Qaeda suspects sentence in Spain last September.

During the trial, prosecutors accused him of helping to prepare a meeting in Tarragona, northeast Spain, in July 2001 at which they said the 9/11 attacks may have been planned. Prosecutors demanded that he be sentenced to a total of 74,337 years for the murder of the 2,973 people killed on 9/11.

But the court decided that there was no proof that he had taken part in the World Trade Center attack, although there was evidence that he had helped think it up, working with a radical cell in Hamburg run by Muhammad Atta, the leader of the 9/11 hijackers.

After Yarkas appealed, however, prosecutors went back through the evidence and decided that it did not stack up. In April, they asked the court to dismiss the conspiracy conviction for lack of evidence.

Mr Hilali, 38, is wanted by Spanish authorities for conspiracy to murder. His extradition was approved by magistrates last June and he lost a High Court appeal last week. He now plans to appeal to the House of Lords - and the decision to quash Yarkas's conviction will give his lawyers an extra argument.

Mr Hilali is said to have spoken on the telephone with Yarkas on four occasions, before and after 9/11. In one wire-tapped call, he said: "They are giving very good classes... we have entered the area of aviation," and "We have slit the bird's throat."

Moussaoui was sentenced to life by a court in Virginia last month after having testified that he was - despite previous assertions and evidence from elsewhere - part of the 9/11 plot. He said that he and Richard Reid, the British shoebomber, had been supposed to hijack a fifth aircraft and fly it into the White House.

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