A unanimous Supreme Court said Monday that undocumented workers who use phony IDs can't be considered identity thieves without proof they knew they were stealing real people's Social Security and other numbers.

The court's decision limits federal authorities' use of a 2004 law, intended to get tough on identity thieves, against immigrants who are picked up in workplace raids and found to be using false Social Security and alien registration numbers.

Advocates for immigrants had complained that federal authorities used the threat of prosecution on the identity theft charge, which carries a two-year mandatory prison term, to win guilty pleas on lesser charges and acceptance of prompt deportation.

"These prosecutions have been taken off the table," said Nina Perales, southwest regional counsel for the Mexican American Legal Defense and Education Fund.

The court, in an opinion by Justice Stephen Breyer, rejected the government's argument that prosecutors need only show that the identification numbers belong to someone else, regardless of whether the defendant knew it.

Breyer said intent is often easy to prove in what he called classic identity theft. "Where a defendant has used another person's information to get access to that person's bank account, the government can prove knowledge with little difficulty," Breyer said.