Most British retailers say they're ready for the switch to the new 12-sided £1 coin but you may still have trouble using it to pay for parking or a train ticket.
The coin goes into circulation on Tuesday. Shops, banks and vending machine operators have had nearly three years' warning.
Large grocery chains such as Tesco (TSCDY) have upgraded their self-service checkout machines. Sainsbury's, another supermarket chain, says its checkouts and shopping carts -- or trolleys -- are ready for the new coins.
London's Tube is also prepared: It spent about £250,000 ($315,000) upgrading more than 1,000 ticket machines.
But try buying a ticket for a suburban train in the British capital and you may not be so lucky. There are still 49 machines on the London Overground network that need to be replaced before they can accept the new coins, a spokesperson said.
And if you're driving, keep a few old £1 coins handy for a while longer.
At least one in eight parking machines will have trouble giving drivers change since they'll have problems sorting the new coins, according to the British Parking Association. The industry group estimates the cost of updating these machines will be around £50 million ($63 million).
"There is a possibility that the new £1 coin may encourage a move towards cashless payments, however there is evidence to suggest that significant numbers of users do not trust new technology such as phone apps," said Kelvin Reynolds, the association's director of policy and public affairs.
Other vending machines and photo booths in the U.K. may also cause problems. About 75,000 machines still need to be upgraded so they can handle the new coin, according to the Automatic Vending Association.
The Royal Mint announced in early 2014 that it was introducing the new coin with security features that would help stamp out fraud and counterfeiting. More than 2.5% of all old £1 coins -- approximately 1 in 40 -- are fakes.
While the big retailers should take the switch in their stride, it will cause a headache for smaller stores. Banks have been telling them to sort the old coins from the new before depositing them.
"We're not particularly happy about [this] because we want to be serving customers, not counting coins," said Chris Noice, spokesperson for the Association of Convenience Stores.
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The new coin is made of two metals: an outer ring of nickel-brass and an inner ring of nickel-plated alloy.
The old coins will cease to be legal tender in October. Banks will continue to accept them after that.
-- CNNMoney's Ivana Kottasova contributed to this report.
