In just over two weeks, the old £1 coin will lose its status as legal tender, meaning shops and businesses will no longer be obliged to accept it.
The new 12-sided coins were released at the end of March this year and are currently in a period of “co-circulation” until 16 October 2017.
However, the Treasury and Royal Mint estimate that some 500 million round pound coins are still out there – down the backs of sofas or in coat pockets.
The Royal Mint said: “We would encourage you to spend, bank or donate your round £1 coins before October 15.
“We have urged businesses and their frontline staff to, where possible, prioritise the new coin when giving customers their change.
“Customers are entitled to ask for their change in any way they wish, but until October 15, businesses can continue to give out the old coin.”
Other amenities, such as vending machines, shopping trolleys, lockers, self-service checkouts, and ticket machines should now have been upgraded so that they accept the new coins.
The coins were switched out after a study found that around one in 30 of the old coins in circulation are fake.
