Undated coins have not entered circulation for more than 300 years
A batch of 20p coins accidentally issued with no date on them could be worth £50 each, say coin dealers.
Tens of thousands of the coins have been produced in error by the Royal Mint at Llantrisant, near Cardiff.
They are said to be the first undated British coins to enter circulation for more than 300 years.
The Royal Mint said the issue had been resolved but wanted to reassure the public that the faulty 20p coins were still legal tender.
But with dealers saying the coins could be worth up to £50 each, anyone finding an undated 20p may be reluctant to put it towards a loaf of bread.
The date on the new 20p was moved from the 'tails' to the 'heads' side when the country's coins were redesigned last year.
The coins without a date were created by accidentally pairing up the new 'tails' side with the old 'heads' - meaning no date appeared at all.
A batch of somewhere between 50,000 and 200,000 of the coins are thought to have entered circulation.
A Royal Mint spokesperson said: "The Royal Mint can confirm that a small number of new design 20 pence coins have been incorrectly struck using the obverse from the previous design, resulting in these coins having no date.
"The issue has now been resolved and the Royal Mint would like to reassure members of the public that these coins are legal tender."
The 20p piece was first struck in 1982
The Tudor Rose design was used continuously on the 20p until 2008
There have been two other coins with errors struck in the decimal era since 1971, but they did not enter circulation.
The first British coin issued with a date in numerals was the English 1551 silver crown of King Edward VI
From 1662 onwards all coins have carried the date of issue
In the mid-19th century, Queen Victoria issued a series of silver florins that were dated in Roman numerals
Source: The London Mint Office
Nick Hart, of specialist coin dealers The London Mint Office, said: "Last year the Royal Mint changed all the designs of our circulating coinage, which is a tradition every 40 years or so.
"And when they did this they struck a new design on one side of the coin and the old design on the reverse of the coin and that's led to one of the sides being undated - which makes them incredibly valuable."
He added: "We believe this extremely rare error will certainly get the public looking at the coins in their pockets again and noticing the excellent new designs launched by the Royal Mint last year on our coinage."
Matthew Dent, 26, from Bangor, Gwynedd, won a competition to design the first new British coin series for 40 years.
The London Mint Office said there had been two other coins with errors produced in the decimal era, but they did not enter circulation.
In 1983 some 2p coins were struck with a die that used the old terminology "NEW PENCE" instead of "TWO PENCE".
And in 1994 a gold £2 commemorative coin was issued with the wrong legend on the Queen's portrait side.