The Bank of England has advised consumers that they have one week left to use the old £20 and £50 paper banknotes before they cease to be legal tender.
After 30 September the Bank will take the old notes out of circulation, and businesses will no longer be accepting them as payment after this date.
The majority of paper £20 and £50 notes have been replaced with the recently issued polymer versions.
But there are still more than £5bn of paper £20 notes featuring the economist Adam Smith, and nearly £6bn of paper £50 banknotes featuring engineers Boulton and Watt, in circulation.
"That’s more than 250 million individual £20 banknotes, and more than 110 million paper £50 banknotes," the BoE said on Thursday.
Threadneedle Street also said that all polymer banknotes carrying a portrait of Queen Elizabeth II will remain legal tender following her death, and the public can continue to use these as normal.
A further announcement regarding existing Bank of England banknotes will be made once the period of Royal mourning has been observed, the Bank said.
What will happen if you still have the old banknotes after 30 September?
After 30 September you will not be able to use the paper banknotes.
However if you have a UK bank account, you will normally still be able to deposit them at your bank, or into your account at a Post Office.
You can exchange certain withdrawn paper banknotes for polymer notes at a limited number of Post Office branches.
This is a new service, and the full list of branches is on the Bank of England's website.
You can also exchange withdrawn banknotes by visiting the central bank, although the Bank said that long queues are forming and that people could wait for more than an hour if they rush to change the money.
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