More than £19 billion in old £20 and £50 banknotes are still in circulation – despite them becoming obsolete within six months.
The paper bills have been swapped over the last two years with plastic replacements.
However, an estimated 775 million old notes are still kicking around.
The deadline to use the notes is September 30th.
From then on, they will not be legal tender.
The £50 note features Alan Turing, the code breaker who cracked the German Enigma machine – helping the Allies win WWII.
And the £20 note shows J.M.W. Turner the English painter.
The Bank of England says it will continue to swap old notes for their face value.
It comes after £5 and £10 notes were replaced over four years ago.
Shoppers can post old notes to the Bank of England in London or deposit them at their local bank.
A Bank of England spokeswoman said:
“All genuine Bank of England banknotes that have been withdrawn from circulation retain their face value for all time.”