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 old and new £1 coins have very different designs but both feature Queen Elizabeth.
The new £1 coin looks more like Europe's €1 coin.
The coin goes into circulation on Tuesday. Shops, banks and vending machine operators have had nearly three years' warning.
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.
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 theAutomatic Vending Association.