A Government push to phase out the old pound coin is being hampered by firms mistakenly returning its 12-sided replacement.
Around half of the coins being delivered to cash centres have turned out to be the new pound coin, slowing efforts to remove the round pound from circulation.
The Treasury has called on businesses to be vigilant when sending coins in as the old pound will cease to be legal tender in less than eight weeks' time.
Andrew Jones, the Exchequer Secretary to the Treasury, urged companies to "maintain the momentum" when it came to returning the old coin.
Visiting Vaultex UK in Dagenham, east London, one of Britain's largest coin storage facilities, Mr Jones said: "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.
"But there is still more to do before the October 15 deadline.
"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.
"We also want cashiers and shopkeepers working at till points, who are truly on the front line of the changeover, to play their part to ensure only new pound coins are given to shoppers in their change."