Consumers have just 100 days to spend £16bn worth of old-style paper banknotes before they are no longer legal tender.
Millions of £20 and £50 notes printed on paper remain in circulation but from 1 October only the newer polymer versions can be used.
The paper £20 note features the economist Adam Smith while the £50 note has portraits of entrepreneur Matthew Boulton and engineer James Watt.
Estimates suggest more than 300 million paper £20 banknotes, and 160 million paper £50 banknotes are still in circulation.
Plastic £50 banknotes featuring a picture of famous Bletchley Park codebreaker and scientist Alan Turing were first issued a year ago, following their introduction for £5, £10 and £20 denominations.
Bank of England chief cashier Sarah John said: “Changing our banknotes from paper to polymer over recent years has been an important development, because it makes them more difficult to counterfeit, and means they are more durable.
“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.
“For the next 100 days, these can still be used or deposited at your bank in the normal way.”