PORTLAND, Maine >> A treasure hunter said Wednesday he has located the wreck of a British merchant ship that was torpedoed by a German U-boat off Cape Cod during World War II while carrying what he claims was a load of platinum bars now worth more than $3 billion.

If the claim proves true, it could be one of the richest sunken treasures ever discovered.

But an attorney for the British government expressed doubt the vessel was carrying platinum. And if it was, in fact, laden with precious metals, who owns the hoard could become a matter of international dispute.

Treasure hunter Greg Brooks of Sub Sea Research in Gorham, Maine, announced that a wreck found sitting in 700 feet of water 50 miles offshore is that of the S.S. Port Nicholson, sunk in 1942. He said he and his crew identified it via the hull number using an underwater camera, and he hopes to begin raising the treasure later this month or in early March with the help of a remotely operated underwater vessel.

"I’m going to get it, one way or another, even if I have to lift the ship out of the water," Brooks said.

The claim should be viewed with skepticism, said Robert F. Marx, an underwater archaeologist, maritime historian and owner of Seven Seas Search and Salvage LLC in Florida. Both an American company and an English company previously went after the contents of the ship years ago and surely retrieved at least a portion, Marx said. The question is how much, if any, platinum is left, he said.

"Every wreck that is lost is the richest wreck lost. Every wreck ever found is the biggest ever found. Every recovery is the biggest ever recovery," Marx said.

Brooks said the Port Nicholson was headed for New York with 71 tons of platinum valued at the time at about $53 million when it was sunk in an attack that left six people dead. The platinum was a payment from the Soviet Union to the U.S. for war supplies, Brooks said. The vessel was also carrying gold bullion and diamonds, he said.

Brooks said he located the wreck in 2008 using shipboard sonar but held off announcing the find while he and his business partners obtained salvage rights from a federal judge. Salvage rights are not the same as ownership rights, which are still unsettled.

Britain will wait until salvage operations begin before deciding whether to file a claim on the cargo, said Anthony Shusta, an attorney in Tampa, Fla., who represents the British government. He said it is unclear if the ship was even carrying any platinum.

"We’re still researching what was on the vessel," he said. "Our initial research indicated it was mostly machinery and military stores."

The U.S. government has not weighed in on the court case yet, and Brooks said he doubts that will happen, since the Soviets eventually reimbursed Washington for the lost payment.

A U.S. Treasury Department ledger shows that the platinum bars were on board, Brooks said, and his underwater video footage shows a platinum bar surrounded by 30 boxes that he believes hold four to five platinum ingots each. But he has yet to bring up any platinum, saying his underwater vessel needs to retrofitted to attach lines to the boxes, which would then be hoisted to the surface by winch.

"Of course there are skeptics," he said. "There’s skeptics on everything you do."

Maritime law is complicated, and there could be multiple claims on the ship’s contents.

After the sinking of the HMS Edinburgh, an English warship carrying Russian gold bullion as a payment to the allies during World War II, England, the U.S. and Russia made claims on the sunken treasure, Marx said. The salvage company was given 10 percent of the prize, while the three countries split the rest, he said.

In other big finds, treasure hunter Mel Fisher made international headlines in 1985 when he discovered a $450 million mother lode of precious metals and gemstones from a Spanish galleon that went down off Florida in 1622.

In another case, a Tampa exploration company has been ordered by the courts to return $500 million worth of treasure from a Spanish warship to Spain. The ship was sunk by the British navy during a battle off Portugal in 1804.

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Associated Press writer David Sharp in Portland, Maine, and researcher Barbara Sambriski in New York contributed to this story.