Hundred of millions of paper bank notes remain in circulation, says the Bank of England, but households are now running out of time to spend them.

The Post Office says it is preparing for a rush of customers wanting to deposit the old-style paper £20 and £50 notes before the end of Friday, when both will lose their status as legal tender.

With more than 257 million paper £20 notes, with a value of £5.1 billion, and 118 million paper £50 notes, with a value of £5.9 billion, thought to still be in circulation people are being encouraged to check that they don't have any tucked away in homes or businesses.

After Friday the paper £20 and £50 notes cannot be spent on the high street or be used to pay for other goods and services.

The Post Office says its 11,500 branches across the UK are preparing for a rush of customers as the final hours to get rid of the notes tick down.

Martin Kearsley, Post Office Banking Director, said: "We’re fully aware that people lead busy lives and some may put off depositing their paper £20 and £50 banknotes until the last moment.

"Postmasters and their staff are on hand to provide that human reassurance that your old notes have been deposited into your bank account and will provide a receipt too. Most Post Offices are open long hours including on Friday."

The old notes are being returned steadily to the Bank of England and replaced with the new polymer notes, which allow for more security features that make them harder to counterfeit.

Alongside being harder to fake, the new style notes are also resistant to dirt and moisture so they should remain in a better condition for longer despite the amount of time they pass between different hands.

What happens after Friday?

After September 30 anyone who finds a forgotten paper note and has a UK bank account will be able to ask to deposit it at their branch. Many Post Office branches will also still be able to accept the withdrawn notes as a deposit to any bank account you can access with them.

The Bank of England will also always exchange any withdrawn notes, and this includes paper notes that have been taken out of circulation in the past.

However this is most usually done through the post and so that exchange is done at someone's own risk, while there are warnings that its counter service, open Monday to Friday, is currently experiencing high demand with long queues and wait times of more than two hours.