Saajid Muhammad Badat testified on Thursday night against Adis Medunjanin, who is suspected of plotting to blow up trains on the New York subway in an attack similar to the July 7 bombings.
Badat, an associate of the shoe bomber Richard Reid, gave evidence in a video recording played in court in public.
His involvement in the trial only became known this week, and forced the Crown Prosecution Service to admit he had given evidence in other terror trials and was rewarded with a two-year reduction in his 13-year sentence.
Now The Daily Telegraph has learnt that the Metropolitan Police tried to keep further details of his testimony secret.
Detectives from Scotland Yard's counter-terrorism unit sought to have sections of Badat's evidence censored and tried to have images, including court sketches, banned from publication.
Officers feared that having such details made public could endanger Badat, who is out of prison and believed to be living in Gloucester.
It is thought that Badat, who was jailed in 2005 over his part in a conspiracy to blow up a plane, has testified in as many as 18 criminal trials.
The Met fears that Badat's evidence in New York will make it clear against whom he has testified previously. It is understood that the US court denied Scotland Yard's requests.
Badat's evidence is believed to shed light on the process of al-Qaeda training. Badat was tutored in Pakistan before his planned attack on a US-bound flight from Amsterdam, from which he pulled out.
Reid went ahead with the plot in 2001 on a Paris to Miami flight, but his shoe bomb failed to detonate.
Badat is not thought to have met Medunjanin or his alleged accomplices, Najibullah Zazi and Zarein Ahmedzay.
Zazi and Ahmedzay have pleaded guilty to their parts in the plot and both testified against Medunjanin this week.
The court has heard that both may have their sentences reduced for giving evidence.