A man who was told a gold coin in his possession was a fake is set to become a millionaire after experts realised it was the real deal.

The collector, who wished to remain anonymous, was convinced his coin was a fraudulent replica of a special $5 coin produced by the San Francisco Mint at the height of the California Gold Rush in 1854.

Coin dealers agreed with him, since only three out of the 268 Liberty Head Half Eagles made were known to have survived into modern times.

But it was soon to be four after the man took it to the Numismatic Guaranty Corporation (NGC), a coin authentication company, whose experts confirmed it was an original.

“It’s like finding an original Picasso at a garage sale. It’s the discovery of a lifetime,” NGC chairman Mark Salzberg said.

“The owner of the coin is a lifelong New England resident who wants to remain anonymous. He was stunned when we informed him that it is a genuine, multimillion-dollar, rare coin.

“He had shown it to a few collectors and dealers at a recent coin show, but everybody said they thought it was a fake because until now there were only three genuine surviving 1854 San Francisco Mint $5 gold pieces known.”

Experts also had to ensure it was not one of the three coins known to already be in existence, after one was stolen in 1968 from the Du Pont family – one of the wealthiest families in America – and never recovered.

“Our initial reaction on examining the coin from New England was utter disbelief that a rarity of this magnitude could still be discovered in this era," said NGC president Rick Montgomery. "But upon seeing the coin in person for the first time it was apparent that the coin is genuine."

Richard Beale, auction valuer at collectables specialists Warwick & Warwick, told The Independent the coin was now likely to be worth “comfortably in excess of a million dollars”.

Mr Beale said the last time one was sold was in 1982 for $170,000, but would sell for far more today at auction.