UP TO 200,000 undated 20p coins that have gone into circulation by mistake – and they could be worth £75 each.
The coins are the first to be produced undated by the Royal Mint for more than 300 years.
Coin experts say the are worth between £50 and £75.
The blunder followed the introduction of new coins last year bearing a fragmented image of the royal
coat of arms on the reverse – or "tails" side – and a new profile of the Queen on the front.
Between 50,000 and 200,000 were wrongly minted using the old version of the Queen's head, which does not have the 2008 date on it.
The mistake went unnoticed and the coins are still legal tender, but their rarity makes them valuable to collectors. A private collecting firm yesterday lifted the lid on the blunder by offering to pay £50 per specimen.
The Royal Mint's last major error was in 1983 when a new 2p coin was wrongly worded with "new pence" rather than "two pence".
And the last time an undated coin entered circulation was more than 300 years ago in 1672, when Charles II reigned.
The mistake happened last year, when all coins from 1p to £1 were redesigned. The 5p and 10p designs had been the same for 40 years, and there is an unwritten convention that images should be updated every four decades to keep them "fresh".
A Royal Mint spokesman said: "We can confirm that last year a small number of new design 20p coins were incorrectly struck using the obverse from the previous design, and as a result these coins bear no date.
"Prior to the new reverse designs launched in April 2008, the date appeared on the reverse side of the 20p.
"As the date has been moved to the obverse side, minting the coin with the new reverse but previous obverse has meant no date appears on the coin.
"We would like to reassure members of the public that these coins are legal tender."
Nick Hart, a coin expert at the London Mint Office, which is urging members of the public to cash in if they find any, said: "Mistakes like this not only fascinate people but also have considerable value significance.
"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."
Nigel Mills, a London-based coin collector, said: "We have heard of these coins selling for as much as £200 at auction.
"They are pretty rare and this kind of thing hasn't really happened before. That's why they are fetching so much when they come up for auction. I've not even seen one myself."
