The unnamed searcher made the discovery in a Scottish river and it is thought to be worth £80,000.
Weighing in at 121.3g, the 22-carat lump of gold is the biggest find of its kind in the UK and it has a small hole in the middle.
It was discovered in two pieces but slots together perfectly, earning it the name The Reunion Nugget.
Lee Palmer, author of Gold Occurrences In The UK, was approached by the mystery finder of the nugget while researching his book.
He said: “This is now the largest nugget in existence in the UK.
“When you look at it, it’s doughnut-shaped.
“There are no impurities in it, it is just pure gold nugget of about 22 carats. It really is a remarkable find.”
The Reunion Nugget was discovered in a mystery location in May this year.
It was found using the method of sniping, which sees gold hunters lying face down in a river while wearing a snorkel and dry suit.
A British enthusiast unearthed the larger piece first, which weighs 89.6g, before finding the other 31.7g half ten minutes later.
Mr Palmer, 50, said: “The man just threw the bigger piece in his bucket with the rest of his stuff – he knew it was big but didn’t realise how big.
“He found the second nugget 30 centimetres away and chucked that in his bucket too.
“It wasn’t until a couple of days later that he had a look at them and realised how big they were and that they fitted together.”
He added: “The hole in the middle could have been caused by a strike off a rock or glacier.
“One mineralogist thought it looked like an entry and exit hole that could’ve been made with a neolithic antler pick, which were used by farmers in the Iron Age.”
Both the finder of the nugget and the owner of the land where it was discovered are keeping their identities secret due to its magnitude.
Mr Palmer hopes it will be purchased by either the National Museum Of Scotland or the Natural History Museum, but legally it may have to be handed over to The Crown Estate.
He believes the fact it is in two pieces should not affect its value.
Mr Palmer said: “From the top you could say it looks like two bits, but when you see it from underneath, it’s a perfect fit.
“It’s like an exact jigsaw, there’s no disputing it.
“Even if you took the largest individual piece, it is still the biggest one in the UK. Add together the second piece and the story behind it and you’ve got something
The current record for the largest gold nugget found in the UK for 500 years was set in 2016.
Called The Douglas Nugget, it weighs 85.7g and was again discovered in a Scottish river by a man in his 40s.
He kept quiet for two years before publicly revealing his find.