The phasing out of old £1 coins is taking longer than expected as businesses keep wrongly returning the new version of the coin.
Cash management company Vaultex said the old round pound will stop being legal tender on October 15 this year but half of the coins being sent back to the banks are the new 12-sided style.
Andrew Jones MP, 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 added: "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."
The new coin was introduced by the Royal Mint in March this year because one in 30 £1 round coins are deemed fake. The new coin is 12-sided, making it harder to copy.
From March until September there is a co-circulation period, during which both versions of the £1 coin will be accepted.
But after October 15, businesses can refuse to accept the old £1 coin. However, they may still be paid into a bank or post office.