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.