The plucky explorer found the precious 85.7g [2.75 Troy ounces] lump while diving in a Scottish river and it is estimated to be worth upwards of £50,000.
Named the Douglas Nugget, it is the biggest gold nugget to have been unearthed in British waters for 500 years.
Gold expert Leon Kirk, from Gold Panning Supplies UK, said: “This is a very exciting and unprecedented find.
“But the nugget’s rarity means it is very hard to put a price on it.
“I would say it is worth at least £50,000 but, as it’s rarer than a Aston Martin or a Faberge egg, a billionaire could easily come along and pay a lot more for it.
“Historically, it is off the Richter scale.”
The Douglas Nugget, which weighs 2.75 Troy ounces [85.7g], is believed to share a similar diameter to that of a 1.6ins golf ball.
It was discovered two years ago by a British father but he has kept it a secret until now.
The man – in his 40s – wishes to remain anonymous due to the magnitude of his find.
The nugget was discovered in a mystery river in Scotland and its exact whereabouts is being kept under wraps.
Breaking his silence, the lucky finder said he came across it by ‘sniping’.
This method sees gold hunters don a dry suit and snorkel before lying face down in a river.
The dad, who has been hunting for gold as a hobby for 20 years, said: “I was following a crack in the bedrock and found around 2g in fine gold.
“This then led to a pocket, where I uncovered the nugget.
“I called over my friend to have a look and we both assumed it to be around 5-7g in weight.
“It wasn’t until I removed it that we realised just how big it was.”
He added: “I took off my glove and picked it up, jumped out of the water and screamed, ’Bingo!’ to my friend.
“We were both stunned and couldn’t believe it. I’ve never seen anything like it in my lifetime.”
The owner is currently unsure what to do with his nugget, but keeping it in a safety deposit box in the meantime.
Mr Kirk hopes it will end up being purchased by a British museum, although legally it may have to be handed over to The Crown Estate.
Dr Neil Clark, author of Scottish Gold: Fruit Of The Nation and curator at The Hunterian, University of Glasgow, believes the shape of the Douglas Nugget could mean there is more gold just waiting to be found close to the mystery location.
He said: “It is difficult to say whether this nugget broke from a larger chunk or not.
“The rounded edges of the piece indicate that it has certainly been in the watercourse for a while.
“The size of the nugget suggests that it probably did not travel far, though.”
But he added: “The fact that we have waited over 500 years for this nugget suggests it may be difficult to find another comparable one in the near future.”
Prior to the Douglas Nugget, the previous record holder for the biggest gold nugget to be found in British waters dates back to 1808.
This was a 59g lump discovered in Cornwall.
In 2016, a 97g nugget was found off the coast of Anglesey by Vincent Thurkettle.
However, experts later concluded that it had originated from Australia and gone down with the steam clipper Royal Charter.
The boat, laden with gold, sank when it was hit by a hurricane while sailing from Melbourne to Liverpool in 1859.