Federal prosecutors want to show for sentencing purposes how the defendant ran a terrorism support cell with global reach. The witness fears arrest in the US, they said.
Babar Ahmad and his co-defendant say the witness should testify in person or the government should bring them to the deposition.
Prosecutors say the court cannot compel his in-person testimony and expressed concerns the defendants would fight their return to the United States if they are allowed to attend the deposition in Britain.
"The government's evidence will show, however, that for years Ahmad surreptitiously ran a terrorism support cell with global reach," prosecutors wrote in court papers on Monday.
They said the cell provided support to Chechen fighters, Taliban and other groups linked to terrorism, including Al-Qaeda.
"Simply put, Ahmad provided to terrorists almost every category of items prohibited by the material support statues and the scope and nature of his conduct was highly organized, complex, far-reaching and worldwide," they wrote.
A hearing on the issue is planned Monday in US District Court in New Haven.
Ahmad and his co-defendant, Syed Talha Ahsan, pleaded guilty in December to supporting terrorists in Afghanistan through websites that sought to raise cash, recruit fighters and solicit items such as gas masks.
Prosecutors didn't name the man they want to testify, but his description matches that of Saajid Badat, a British citizen whose videotaped testimony was shown at the 2012 trial of a man convicted in a foiled 2009 plot to attack the New York City subway system. Badat is testifying in New York City at the trial of Osama bin Laden's son-in-law.
Badat backed out and was sentenced to 13 years in prison that was later reduced to 11 years to reward him for his cooperation in terrorism investigations.
The witness is expected to testify that he moved on from Ahmad and was trained by al-Qaeda members for the shoe bomb plot, according to prosecutors.
Attorneys for Ahmad and Ahsan say the witness has an incentive to lie, arguing he's receiving extensive benefits through his cooperation. Prosecutors say false testimony would violate his cooperation agreement and the defense will have a chance to cross examine him.
