A federal judge in New York has delayed the first civilian trial of a former Guantanamo detainee until Wednesday as prosecutors worried about losing a key witness, The New York Times reported.

Opening arguments will set the stage for a trial that could take several months and test President Barack Obama's efforts to shut the detention facility at the US naval base at Guantanamo Bay, Cuba and move terror suspects into the civilian justice system.

The Times said prosecutors had sought to postpone the trial, which had been due to begin Monday, because US District Judge Lewis Kaplan had not ruled on their request to use a controversial witness in their case against Ahmed Khalfan Ghailani.

Ghailani's lawyers opposed using Hussein Abebe as a witness because the government learned about him during what they say was coercive interrogation and torture of their client while in CIA custody.

Federal attorneys said Abebe would testify that he sold TNT to Ghailani which was later used to bomb the US embassy in the Tanzanian capital Dar es Salaam. Ghailani, a Tanzanian man, is accused of playing a key role in the killing of 224 people when that embassy and another in Kenya were blown up in 1998.

"This is a giant witness for the government," Assistant US Attorney Michael Farbiarz recently told the judge, according to the Times. "There's nothing bigger than him."

Prosecutors say Abebe was cooperating voluntarily and his decision to do so was only remotely linked to Ghailani's interrogation.

On Wednesday, Kaplan whittled down a huge pool of prospective jurors, whose anonymity will be guarded throughout the trial.