Companies are sending back the new 12-sided £1 coins instead of the old ones by mistake, experts have warned.
Efforts to phase out the old coins are being hindered as around half of those sent back are the new ones, cash-processing firm Vaultex has said.
The old-style coins will become obsolete on 15 October 2017.
The new £1 coin became legal tender in March and was introduced to tackle the growing number of counterfeits.
It is estimated that around one in 30 of the old-style pound coins in circulation are fake, according to the Royal Mint.
The new coin has a number of features that make it considerably more difficult to counterfeit. It is made from two metals, the outer gold ring of nickel-brass and the inner section of nickel-plated alloy.
It also has an image like a hologram that changes from a “£” to the number “1” when the coin is seen from different angles.
MP Andrew Jones, Exchequer Secretary to the Treasure, said there is more to be done before the deadline in October.
“There has been a fantastic effort from both the public and businesses in returning more than one billion old round pounds, and I thank everybody involved in this process so far.
“Businesses must remain vigilant when returning coins and ensure old and new coins are organised in separate packaging to make the sorting process quicker and easier.”
