Shoppers have the weekend to spend their old round £1 coins before they are officially withdrawn, but many shops will go on accepting them.
Businesses can refuse to take them from Monday when they will no longer be legal tender.
It is estimated that up to 450 million of these coins are still in the public's hands despite the new 12-sided coin entering circulation in March.
Some supermarkets and other stores said they would give shoppers some grace.
The latest is The Entertainer toyshop chain, which said it would continue to accept round £1 coins until Christmas.
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 round £1 coins were launched on April 21, 1983, to replace £1 notes. The Royal Mint has produced more than two billion round pound coins since that time.
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
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