"It means that the U.S. can do to anyone what they did to Maher," said Maria LaHood, a senior attorney with the U.S.-based Centre for Constitutional Rights.
"They can do it to anyone, to any foreign citizen, and use the immigration process as a guise, basically, to send someone to be tortured."
Arar was labelled a member of Al Qaeda when he switched planes at New York's Kennedy Airport in 2002 as he returned to Canada from vacation.
He was eventually released without charges and he returned to Canada where a judicial inquiry cleared him of any terrorist links and Ottawa awarded him compensation of $10.5 million.
The U.S. Court of Appeals in New York ruled today that Arar's claims that it was a violation of due process to send him to Syria could not be heard in federal court. The court concluded that adjudicating the claims would interfere with sensitive matters of foreign policy and national security.
"It's a quite sweeping and reprehensible opinion," said LaHood. ``It's quite sweeping in how much deference it gives to the U.S. government."
LaHood spoke with Arar and said he is equally taken aback by the decision.
"He was not only disappointed too, but outraged," she said.
"He's rightfully angered that he cannot get justice, and that not only can he not get justice, but that his being sent to torture has now been in vain because he can't even stop the government from doing it to someone else."
The decision also said that Arar, as a foreigner who had not been formally admitted to the U.S., had no constitutional due process rights.