Roughly the size of a golf ball, it was discovered by an anonymous treasure hunter while snorkelling in a Scottish stream.
Named the Douglas Nugget, the piece weighs 2¾ troy ounces, or 85.7g, with a diameter of just over 1½in.
Experts say it is 500 years since a similar nugget has been found in Britain, although there could be more where this one came from.
For this reason, the location of the river has been kept under wraps to prevent a modern gold rush.
The man behind the find has chosen to keep his identity secret but has been hunting for gold as a hobby for 20 years.
He uses the snorkelling technique known as sniping to scour riverbeds for traces of gold.
He said: “It wasn’t until I removed it that I realised just how big it was. I jumped out of the water and screamed, ‘Bingo’. I couldn’t believe it. I’ve never seen anything like it.”
He is keeping it in a safe deposit box while he decides what to do with it.
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.”
