The days of spending paper banknotes in the shops are numbered: 100 days, to be exact - so start hunting at home.
Remaining paper £20 or £50 notes should be spent or deposited by the end of September, the Bank of England said.
An estimated 163 million paper £50 banknotes and about 314 million £20 paper notes were still in circulation, the Bank said.
These notes are being replaced with plastic versions, just like the £5 and £10 note, which are more durable.
The Bank said the newer, polymer notes were also harder to counterfeit.
"The majority of paper banknotes have now been taken out of circulation, but a significant number remain in the economy, so we're asking you to check if you have any at home," said the Bank's chief cashier, Sarah John, whose signature is on the new notes.
From October, people with a UK bank account should still be able to deposit the paper notes into their account or at the Post Office, but spending them will be impossible.
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