There are fears the golf-ball sized 87.5g nugget – found by anonymous gold hunter – could spark a gold rush of fortune-seekers flooding the area.
Named the Douglas Nugget, it was discovered two years ago but its lucky owner has kept his story secret until now.
Experts value it at £50,000 and say it is the biggest nugget found in British waters for 500 years.
Breaking his silence, the man who found it – a dad in his 40s – said he came across it by “sniping”.
The technique involves wearing a dry suit and snorkelling along a shallow river.
The nugget was discovered using a method called ‘sniping’ on a Scottish riverbedThe hobbyist, who has spent 20 years hunting for gold, 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 nugget is safely locked up in a deposit box as he is still undecided what to do with it. And the location where it was discovered is a closely guarded secret.
Gold panning expert Leon Kirk said: “This is a very exciting and unprecedented find.
“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.”
There are three “gold belts” in Scotland — around the Lowther Hills near Wanlokhead and Leadhills in Galloway; Tyndrum in Perthshire; and at Helmsdale in Sutherland.
Dr Neil Clark, author of Scottish Gold: Fruit Of The Nation, believes there could be more gold around the secret area where the Douglas Nugget was found.
He said: “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.