Rights to fish a section of the River Severn which date back to the 6th Century are up for sale.
The Free Fishery of the Severn belonged to the ancient Welsh kings of Glamorgan and Gwent, and stretch three miles (4.8km) from the river bank in Newport to Portishead, near Bristol.
The fishery is being sold on behalf of an unknown south Wales owner.
The Canal and River Trust said the fishery was a “remarkable piece of history”.
A guide price for the lot has yet to be revealed, but in 2000 the Inland Revenue Valuation Office valued the fishery at £50 per acre.
John Ellis, national fisheries and angling manager at the Canal and River Trust, told BBC Wales’ Good Morning Wales programme: “It’s a remarkable piece of history.
“I’ve never come across anything like this in my 30-plus years working on fisheries.”
The fishery pre-dates the Magna Carta, a 1215 act which prevented the Crown giving away fishing rights to those it favoured.
It is therefore not bound by laws which protect public fishing rights.
“I can only think of one other example like this, around the river Towy, where there’s a section of the estuary where fishing rights are in private hands,” Mr Ellis said.
The estuary contains a mixture of salmon, sea trout and eels, the fishing of which Mr Ellis said would be subject to the same restrictions as any other area of sea.
He added: “It could be a really valuable thing. On the other hand it doesn’t give the (new owners) carte blanche to do things that are no longer legal. It will still be regulated if it involves sea trout or salmon.
“There may well be restrictions on the amount of fish caught.
“But to own something so rare will have a value in itself. Who knows what the price could end up being.”
About a quarter of the rights to fish in the River Severn are owned by angling groups.
Individuals need licences from the Environment Agency if they are rod fishing for salmon, trout, eels or coarse fishing – but not for marine fish.
The public have a right to fish in the sea below the mean high water mark of tidal waters, and anyone can fish either from the bank or by boat.
Paul Fosh, of Newport-based Paul Fosh Auctions, called the fishery “extremely valuable”, and added: “The term ‘unique’ is often overused but in this case it can be used without any question.
“I am confident in saying that there is no other stretch of river like this anywhere else in the world.
“We expect to get an exceptional amount of interest from a number of different parties in this fishery.”
Mr Fosh said the new owner would be able to sell permits or licences for others to fish, with interest anticipated from people looking to organise fishing trips.