The Bank of England has refused to yield to pressure from protest groups about its use of animal-derived products in bank notes, saying that it will not pull any of the existing £5 notes from circulation and will print the £10 notes as planned.

“The Bank was not aware of the presence of animal-derived products when it signed the contract with its supplier for the £5 and £10 banknote polymer,” the Bank said in a statement on Wednesday.

“When the Bank discovered the presence of these products, its first step was to alert the public and subsequently has been treating the concerns raised by members of the public with the utmost seriousness,” it added.

It said that after careful consideration it has concluded “that it would be appropriate to keep the £5 polymer note in circulation and to issue the £10 polymer note as planned, in September”.

Vegans and vegetarians voiced outrage after the Bank of England said last year that the plastic fivers are made with tallow - a substance derived from fat and used in candles and soaps.

A petition garnered more than 100,000 signatures and stated that the fact that the notes contain the tallow, “is unacceptable to millions of vegans & vegetarians, Hindus, Sikhs and Jains in the UK”.

Innovia, the company that makes the banknotes, said that it had obtained the animal fat through a supplier, which it declined to name. The company said it used the substance to give the notes their anti-static and anti-slip properties, and pointed out that thousands of products contain tallow. It could not confirm which animals the fat had come from.

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On Wednesday the Bank said that it had instructed polymer suppliers to investigate alternative options to the inclusion of animal-derived products in the manufacturing process of the £20 polymer notes which are due to enter circulation in 2020.

“Those alternatives could for example include the use of palm oil or coconut oil as a substitute,” it said. “Suppliers have indicated that a plant-based alternative should have no impact on the quality of banknote production, though this would still need to be validated by production trials over the next few months.”

Professor David Solomon, who developed the first polymer bank note in circulation in Australia in 1988, last year said that the £5 notes contain “trivial” amounts of tallow.

He said polymer notes were extremely hard to forge and had a lot more benefits for the consumer than previous paper notes.

“It picks up less drugs than paper notes and you don’t chop down trees,” he said. “It’s more hygienic than a paper note by a long way.”

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The first of 440 million plastic £5 notes went into circulation in September. The notes, featuring Winston Churchill, fit in cash machines like paper ones, but are considerably more durable as well as being cleaner and harder to counterfeit.