By Marty Graham

SAN DIEGO (Reuters) - A court ruling clears the way for hundreds and perhaps thousands of immigrants improperly expelled to Mexico from Southern California to be allowed to return to seek legal U.S. residency, an official with a civil rights group said on Saturday.

Attorneys from the American Civil Liberties Union contended in a lawsuit on behalf of roughly 10 plaintiffs expelled to Mexico that the immigrants were tricked or coerced into signing their own expulsion orders.

U.S. officials and the ACLU reached a settlement last August to prevent immigration authorities from pressuring undocumented immigrants in Southern California to sign off on their expulsions to Mexico. The agreement laid the groundwork for a broader class of expelled immigrants than the original plaintiffs to potentially return to the United States.

U.S. District Judge John Kronstadt in Los Angeles on Thursday approved the ACLU's request to broaden the class of plaintiffs to include immigrants improperly expelled between June 2009 and August 2014, said Anna Castro, a spokeswoman for the ACLU of San Diego and Imperial Counties.

The settlement is limited in scope, applying to immigrants expelled from Southern California, because the ACLU's allegations of violations of due process centered on that region. But rights groups have hailed it as a victory.

The judge ruled that the class of immigrants who could return was comprised of those with reasonable grounds to believe they might have been allowed to stay in the United States if they had gone before a federal immigration judge.

As a result, hundreds and possibly thousands of immigrants expelled to Mexico from Southern California are eligible to return and present their case in immigration court for legal U.S. residency, Castro said.

The expulsion order U.S. authorities placed before these immigrants is known administratively as a "voluntary return," and ACLU attorneys said immigrants who signed it often were not informed they would be barred legal re-entry for 10 years.

"One of our plaintiffs is a mom who was detained with her 11-year-old son, a U.S. citizen who suffers from Asperger's Syndrome," Castro said. "She was told her son would be put into foster care if she didn't agree to sign the papers."

The Department of Homeland Security has said U.S. agencies agreed to a settlement with the ACLU so they could "supplement their existing procedures to ensure foreign nationals fully comprehend the potential consequences of returning voluntarily to Mexico."

(Editing by Alex Dobuzinskis and Michael Perry)