Babar Ahmad has been held at Long Lartin prison for seven years on alleged terror charges.
During a Westminster Hall debate on Thursday Ms Lucas said she understood that evidence collected by the Metropolitan Police had been given to US prosecutors instead of being handed to the Crown Prosecution Service (CPS) to investigate whether he had a case to answer.
The Green MP said: “In a shocking turn of events last night, Ahmad’s lawyers received a letter from the CPS admitting for the first time that the CPS were never given the evidence that was sent to the US, save for what they call ‘a few documents.’
“So astoundingly, while we had previously been led to believe that the CPS had seen all of the evidence and judged it to be insufficient for a case here in the UK, we now get a confession that CPS hadn’t seen all of the evidence, let alone investigated it properly.”
She said that if the CPS knew all along that it hadn’t been given all of the evidence “yet they have let Babar Ahmed languish in a maximum-security prison” it was “absolutely appalling” and raised serious questions.
Ms Lucas said the CPS must obtain all of the files from the US authorities and review them completely. She also called for a public inquiry.
A spokesman for the CPS said it was sent some of the evidence gathered by the US authorities “as is normal procedure in extradition matters.
“We were never in possession of all the US evidence and have never claimed otherwise.”
He added that at the time the advice was given the CPS was aware of the “nature of the evidence in the possession of the US.”
Mr Ahmad’s father Ashfaq Ahmad said: “It is quite shocking to learn that the CPS made no effort to examine the evidence seized from Babar’s home but instead simply outsourced our criminal justice system to the US.
“Had it examined the evidence, then it could quite easily have prosecuted Babar in the UK.”