By Jonathan Stempel

(Reuters) - A federal appeals court on Monday said a cache of exceptionally rare gold coins stolen from the U.S. Mint in the 1930s belongs to the U.S. government, not the Pennsylvania family that possessed it for decades.

By a 9-3 vote, the 3rd U.S. Circuit Court of Appeals in Philadelphia said Joan Langbord and her sons Roy and David cannot keep the 10 "double eagle" 1933 $20 gold pieces, estimated to be worth several million dollars each.

Monday's decision could end a decade-long battle that began after the Langbords, heirs to late Philadelphia jeweler Israel Switt, found the coins in a safe deposit box and asked the Mint to authenticate them, only to have them seized in 2004.

"The Langbord family fully intends to seek review by the Supreme Court of the important issue of the unbridled power of the government to take and keep a citizen's property," its lawyer Barry Berke said.

First Assistant U.S. Attorney Louis Lappen said: "This ruling properly establishes that the United States is the lawful owner of the 1933 Double Eagle gold coins."

The coins were among 445,500 double eagles struck by the Philadelphia Mint, but never circulated as President Franklin Roosevelt took the United States off the gold standard.

Most were melted down but some were smuggled out, including one sold to Egypt's King Farouk and later in a 2002 auction for $7.6 million.

The government said some coins were stolen by the Mint's cashier with help from Switt, the father of Joan Langbord.

Though Switt was not charged, the government said the Langbords were "the family of a thief" and should not benefit.

In July 2011, a jury said the government could keep the coins, and in August 2012 the trial judge agreed.

But in April 2015, a three-judge appeals court panel ruled for the Langbords, saying the government missed a 90-day deadline after they formally claimed the coins to seek a forfeiture under the Civil Asset Forfeiture Reform Act.

In Monday's decision, however, Circuit Judge Thomas Hardiman said that deadline did not apply, and the government was trying merely to repossess its own property.

"A seizure alone does not initiate a forfeiture proceeding because it does not implicate a transfer of legal title," he wrote.

The full appeals court also refused to grant a new trial, saying errors at the 2011 trial were harmless.

Circuit Judge Marjorie Rendell, who wrote the April 2015 decision, dissented.

She said the majority rendered CAFRA's protections "largely meaningless" by accepting the government's "say-so" that it owned the Double Eagles and had no intention of forfeiting them.

The case is Langbord et al v. U.S. Department of the Treasury et al, 3rd U.S. Circuit Court of Appeals, No. 12-4574.

(Reporting by Jonathan Stempel in New York; Editing by Bernard Orr)