A former New York policeman found guilty of plotting to kidnap, torture, kill and eat women has been ordered free by a federal judge who overturned his conviction.

The sensational trial of 30-year-old Gilberto Valle, dubbed the "cannibal cop", gripped the Big Apple and the tabloid press last year, exposing an underground network of cannibal fantasists.

Valle scoured websites catering to the gruesome fetishes of so-called "death-porn" enthusiasts and detailed his plans to abduct women, boasting that he would torture, kill and eat them.

He drew up a detailed list of his potential victims, including friends and his wife, the mother of his child, who eventually turned him in to the FBI and testified against him in court.

Valle had spent 21 months in jail.

But in a 118-page ruling released, US District Court Judge Paul Gardephe said that however disturbing Valle's "deviant and depraved" fantasies may have been, no actual crime took place.

"No one was ever kidnapped, no attempted kidnapping ever took place, and no real world, non-Internet based steps were ever taken to kidnap anyone," he wrote.

"Despite the highly disturbing nature of Valle's deviant and depraved sexual interests, his chats and emails about these interests are not sufficient -- standing alone -- to make out the elements of conspiracy to commit kidnapping."

At a brief hearing in a Manhattan court today, where prosecutors said they would appeal, the judge ordered Valle released on $100,000 bond.

He ordered him to live at his parents' home until the appeal is heard, wear an electronic monitoring band, be barred from using the Internet and undergo psychological assessment.

His legal team told reporters they were "thrilled."

"It validates what we have been saying since the very beginning of this case -- he is guilty of nothing more than very unconventional thoughts," said lawyer Julia Gatto.

"As Judge Gardephe has validated, we don't put people in jail for their thoughts," she added.

Valle's trial in February and March 2013 transfixed the tabloid press and generated global headlines.

While Valle's lawyers argued that the policeman had merely been guilty of sick online fantasies, prosecutors said he had "crossed the line" into reality and was actively planning to act out his online plot, even if no women were ever harmed.

An FBI agent testified to finding hundreds of downloaded pages on Valle's computer about rape, torture and cannibalism.

He was convicted in March 2013 of conspiracy to kidnap and a second count of illegally using a police database to compile a list of potential victims.

Gardephe upheld the second conviction, which was punishable by a maximum of one year in prison.