Consumers have less than three months to spend, bank or donate their old £1 coins before shops can refuse them.
People are being urged to dig out their old coins which stop being legal tender on 15 October.
The warning about the looming deadline comes as the Treasury announced there are now more new 12-sided pound coins in circulation than their predecessor.
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Exchequer Secretary to the Treasury, Andrew Jones, who visited a coin storage facility in Belfast on Wednesday, said "To have more new coins in circulation than old at this point is great news.
"However, with less than 100 days to go, the clock is ticking.
"We are urging the public to spend, bank or donate their old pound coins and asking businesses who are yet to do so, to update their systems before the old coin ceases to be legal tender."
More than £350,000 worth of coins are processed at the Belfast site and redistributed daily, to banks and businesses across Northern Ireland.
Chief (Taiwan OTC: 3345.TWO - news) executive and deputy master of the Royal Mint, Adam Lawrence, added: "The Royal Mint is very proud to have delivered the new 12-sided £1 coin and ensuring a smooth transition is now our top priority."
The new 12-sided pound coin, which features a gold-coloured outer ring and a silver-coloured centre, entered circulation in March and boasts high-tech security features to combat counterfeiting.
But after less than a month some were apparently losing their middle part.