The paper £5 note will cease to be legal tender this weekend but the Bank of England (BoE) has warned there are millions of notes still in circulation.

A new fiver, made out of polymer and with Winston Churchill replacing social reformer Elizabeth Fry's portrait on it, was introduced in September and while millions of new notes have been printed, according to BoE estimates approximately 150 million of £5 paper notes are still in circulation.

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That means each UK adult has around three of them, in their wallets, piggy banks or stashed at the back of their sofas.

The notes will stop being legal tender at midnight on Friday night (5 May) and shops across the country will be legally allowed to refuse them, but banks are expected to continue to exchange them for a few months.

"We'll continue to accept them from our customers, either exchanging them for the new polymer note, or depositing it into their account, whichever they prefer," said a spokesman for Lloyds Banking Group.

Similarly, a spokesperson for Royal Bank of Scotland added: "After the note goes out of circulation, customers will still be able to bring in their old £5 notes for exchange at one of our branches. Non-customers will be directed to their own bank."

The BoE, however, will continue to exchange the soon-to-be old notes at any time in the future in its offices at Threadneedle Street in the City.

Britain's central bank said the switch to longer-lasting notes will save approximately £100m, although the move could see shops and banks ending up with a bill of up to £236m as ATMs, vending machines and self-service machines will have to be recalibrated or replaced.

Northern Ireland introduced plastic notes in 1999, when Northern Bank – now known as Danske Bank – put two million notes in circulation to mark the new millennium, while plastic notes have been available since March last year in Scotland, albeit on a very limited basis.