Efforts to phase out the old £1 coin are being hampered by companies who are returning the new 12-sided replacement by mistake.
The old round pound ceases to be legal tender on 15 October, with a billion taken out of circulation already.
But the removal process is being slowed down because about half of the £1 coins sent back are actually the new style, cash management company Vaultex says.
Treasury minister Andrew Jones said "businesses must remain vigilant".
The new coin is being introduced because approximately one in 30 pound coins currently in circulation is a fake, according to the Royal Mint and the 12-sided version is designed to be harder to counterfeit.
Mr Jones, Exchequer Secretary to the Treasury, 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 15 October deadline."
He continued: "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."
Cashiers and shopkeepers working at till points must also "play their part to ensure only new pound coins are given to shoppers in their change", he added.
Why the new coin is harder to counterfeit
- 12-sided - its distinctive shape means it stands out by sight and by touch
- Bimetallic - The outer ring is gold coloured (nickel-brass) and the inner ring is silver coloured (nickel-plated alloy)
- Latent image - it has an image like a hologram that changes from a "£" symbol to the number "1" when the coin is seen from different angles
- Micro-lettering - around the rim on the heads side of the coin, tiny lettering reads: ONE POUND. On the tails side, you can find the year the coin was produced
- Milled edges - it has grooves on alternate sides
- Hidden high security feature - an additional security feature is built into the coin, but details have not been revealed
The government estimates about a third of the £1.3bn worth of coins stored in piggy banks around the UK are the old £1 style.
Some of those returned by the public will be melted down and used to make the new version.
After October 15, businesses can refuse to accept the old £1 coin. However they may still be paid into most high street banks, provided the bearer is an account holder.
As arrangements will vary from bank to bank, the public are recommended to check with them in advance.
But to avoid any inconvenience, people are being urged to spend or deposit their old £1 coins before that date.
Pound match scheme
International Development Secretary Priti Patel urged people to donate old pound coins to one of more than 20 UK charities chosen to take part in the UK Aid Match programme.
The government pledges to match every pound.
Ms Patel said: "The generosity of the UK public is one of the things that makes Britain truly great.
"By getting rid of old change, the public can help to double the amount great causes receive, ensuring their support goes even further."
Donors can see which charities are taking part in the scheme by looking for the UK Aid Match logo, featuring the Union Flag.
A joke about the changeover was voted the funniest of the Edinburgh Festival Fringe.
Ken Cheng quipped: "I'm not a fan of the new pound coin, but then again, I hate all change."
