The Bank of England in September unveiled an innovative £5 note made of thin, flexible plastic. The idea, the bank said, was to introduce a longer-lasting, waterproof currency that would be difficult to counterfeit.

But, it turns out, the bill bearing Sir Winston Churchill’s image also contains trace amounts of a substance that is proving to be highly controversial: Animal fat.

The Bank of England confirmed via Twitter on Monday that its £5 notes contain “a trace of tallow,” an animal fat product commonly used as an industrial lubricant and sometimes found in candles, soaps and other household items.

“There is a trace of tallow in the polymer pellets used in the base substrate of the polymer £5 notes,” the bank Tweeted on Monday.

The backlash was immediate: “How sick, unnecessary & prehistoric,” wrote one user.

“Not cool at all,” added another. “I go to a lot of trouble to avoid animal products. Going to start refusing them.”

In the case of the polymer notes, small beads of tallow are likely used to help the currency feed smoothly through machines, to ensure that it doesn’t jam or get stuck, said Alan Sentman, a chemist and lab manager at Polymer Solutions, a testing lab in Christiansburg, Va.

But, he added, tallow isn’t exclusive to animal fat. It can also be derived from vegetable oils, cocoa butter and other non-animal sources.

“They absolutely could move to a pure vegetable alternative — there’s no reason that wouldn’t work,” Sentman said. “It’s just a little bit cheaper to do with animal fat.”

A petition on the site calling on the bank to cease the use of animal products in its currency had more than 11,600 signatures as of Tuesday morning.

“When you consider that beef tallow is a co-product of a cruel and violent industry that kills millions of cows every year and that is one of the largest producers of greenhouse-gas emissions in the world, the decision to use animal fat in the new British five-pound note truly doesn’t make sense,” a spokeswoman for People for the Ethical Treatment of Animals said in an email.

The Bank of England, which has already printed 440 million new £5 notes, has not announced plans to change the make-up of its bills. The bank is starting to phase out its use of cotton paper in currency. It says it plans to begin issuing plastic £10 notes next summer, and £20 notes by 2020.

Australia was the first to introduce polymer currency, in 1988. A number of countries, including Canada, Singapore, Chile and Nepal have followed suit.

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