The panel also said immigrants held more than a year should get bond hearings every six months where the federal government must show why they should remain locked up.

"It substantially decreases the likelihood people will get lost in the system for years on end because there will be some examination of why the person is still incarcerated," said Ahilan Arulanantham, deputy legal director for the American Civil Liberties Union of Southern California, which represented plaintiffs in the case.

The government is reviewing the court's decision, said Nicole Navas, a spokeswoman for the Department of Justice.

The ruling came after years of litigation over whether immigrants unable to make bond or not initially granted bond are entitled to additional bond hearings. The ACLU in Southern California filed the lawsuit in 2007 on behalf of Mexican immigrant Alejandro Rodriguez, who was detained for several years without a bond hearing.

For several years now, the hearings have been taking place but the ruling expands which immigrants should get them. Arulanantham said the appellate decision could increase the number of immigrants who get the hearings by roughly a third.

An ACLU study released last year found that 69 percent of 1,680 detained immigrants in Southern California who got the bond hearings over an 18-month period were granted bond.