A New York jury has found Osama bin Laden’s son-in-law, former Al-Qaeda spokesman Suleiman Abu Ghaith, guilty on three counts of conspiracy to kill Americans and supporting terrorists.

The 48-year-old preacher from Kuwait, whose three-week trial has been the most high-profile Al-Qaeda case to reach a US federal court, now faces life behind bars.

Abu Ghaith stood impassively as the court clerk read out the verdict, declaring him guilty of conspiracy to kill Americans, conspiracy to provide material support and providing material support to terrorists.

Judge Lewis Kaplan said Abu Ghaith, who was brought to the United States only last year after being detained in Jordan, would be sentenced on September 8.

Campaigners seized on the trial as proof that terror detainees can face swift justice in civilian courts, as pressure builds to close the Guantanamo Bay prison camp.

“There is to be no demonstration by anyone in the court room of any kind,” Kaplan told the packed room after the 12-person jury took just four hours of deliberations to return the unanimous verdict.

Afterwards, Abu Ghaith touched his heart in thanks, smiling at his lawyers.

Defence lawyer Stanley Cohen told reporters that he would appeal the verdict, accusing the judge of being “coercive,” and taking issue with instructions to the jury and Kaplan’s decision not to allow presumed 9/11 plotter Khalid Sheikh Mohammed to testify in court.

Defence lawyer Stanley Cohen. Photo: Getty

He said his client was “stoic” and “at ease”.

“He has confidence that this is not the end, but the beginning. We think there are a number of compelling issues for an appeal,” Cohen said.

US prosecutors had alleged Abu Ghaith was an Al-Qaeda insider and Bin Laden’s right-hand man and inspired a new generation of terrorists as the group’s main messenger after the 9/11 attacks.

Abu Ghaith denied the charges, but is most famous for appearing with Bin Laden in a video on September 12, 2001 claiming the attacks on US targets that killed nearly 3000 people the day before.

In follow-up videos, he threatened a “storm of airplanes,” proof the government said that he was implicated in the December 2001 plot to blow up a transatlantic flight from Paris with a shoe bomb.

The prosecution urged the jury to find the defendant guilty on all three counts, referring repeatedly to what they called “overwhelming” evidence against him.

Video and audio clips, his confession to an FBI agent en route to the United States last year and his testimony on the stand all proved his guilt, they said.

Abu Ghaith fled Afghanistan for Iran in 2002. He was arrested in Turkey in 2013 and sent to Jordan, where he was transferred to US custody.