Drivers are being warned of a parking nightmare when the new £1 coins are introduced next month.
The new twelve-sided coin is due to come into circulation on March 28, with the old coins being gradually withdrawn from October onwards.
But while both will be legal tender during the period of overlap, drivers are being warned that they may face chaos when trying to park.
The least common (and most valuable) £1 coins
Tens of thousands of pay and display machines - around a quarter of the UK's total - are not expected to be ready in time.
According to the British Parking Association, rural areas are the most unprepared, with many machines too old to be modified for the new coins.
It says that some councils are considering making parking machines cashless instead, as introducing tap-and-go technology could work out cheaper than changing the shape of the coin slots.
The cost of adapting all the pay-and-display machines in the country could be as much as £50 million, it adds.
Vending industry complains over switch to new £1
"Many operators already offer flexible payment methods including credit cards with a small number of motorists using mobile phone apps," says the BPA's director of policy and public affairs, Kelvin Reynolds.
Similarly, many vending machines may not be updated in time - as many as 40%, according to the Automatic Vending Association.
"Machines run by private companies are likely to be slower to adapt as they might need to get permission to do the modification, which is more time consuming," chief executive Jonathan Hart tells the Daily Telegraph.
New 12-sided £1 up on eBay - but are they legal to buy and sell?
The amusement industry is also worried about the cost of adapting gaming machines to the new coins.
"We can't pass on any cost in the amusements industry. We're constrained not only by the coinage itself but also by having a maximum stake," says chief executive of industry body Bacta. "So any time there's a cost increase, it just comes straight off the bottom line."
Meanwhile, a new survey from Mastercard has revealed that as many as 40% of the public is still unaware of the changeover. It estimates that the public could lose a whopping £1.1 billion by failing to spend all their coins in time.