Shoppers have until midnight to spend their old round £1 coins before they are officially withdrawn.
But with up to 450 million estimated to be still in the public’s hands, many retailers have said they will continue to accept them for a limited period.
Businesses can refuse to take them from Monday when they will no longer be legal tender.
The round coin, launched in April 1983, is replaced by the new 12-sided coin that entered circulation in March.
Some supermarkets and other stores said they would give shoppers some grace.
The Entertainer toyshop chain, which said it would continue to accept round £1 coins until Christmas.
Iceland and Poundland have said they will continue to accept the old round pound until 31 October, while Tesco has said it will continue accepting the old-style coins for a week after the 15 October deadline.
- Where else can you spend your old pound coins?
Coins down the back of the sofa?
With the deadline to use the old £1 coin just days away, here are some of the places to look for them:
- Supermarket bags
- Gym bag
- Pockets of the coat(s) you wore last winter
- Down the back of the sofa
10 places to find your old £1 coins
Some shoppers have been angered that shops have continued to give round pounds as change in the run-up to the deadline.
Consumers and businesses can still bank these coins beyond Sunday night’s deadline.
Martin Kearsley, banking services director at the Post Office, said: “Thanks to an agreement with all UK High Street banks, everyone can deposit old pound coins into their usual High Street bank account at their local Post Office branch.”
Some other payment services might not be ready for the deadline.
The British Parking Association has said it is confident that the majority of parking machines are ready or will be ready to accept the new £1 coin, but the Automatic Vending Association said it believed all machines owned by its members were now accepting the new coin.
The Royal Mint has produced more than two billion round pound coins since its launch.
The new £1 coin: Vital statistics
Thickness: 2.8mm – thinner than old coin
Weight: 8.75g – lighter than old coin
Diameter: 23.43mm – larger than old coin
Number to enter circulation: 1.5 billion – about 23 per person. Old £1 coins will be melted down to make new ones
Outer ring: gold-coloured, made from nickel-brass
Inner ring: silver-coloured, made from nickel-plated alloy