It’s enough to put you off your six-step moisturising routine. Rat poo, arsenic, cyanide and even human urine are all among the not-so desirable substances that have been found in fake perfumes and makeup being sold online in increasing quantities, police have warned.

Last year, the City of London’s Police Intellectual Property Crime Unit dismantled a criminal operation suspected of importing and selling counterfeit goods, seizing a shipping container full of what they believe to be more than 4,700 counterfeit versions of one of the UK’s most popular beauty brands – with products including foundation, bronzer, lip gloss, eye shadow and eyebrow pencils.

Fake makeup is typically manufactured in the Far East – China is one of the main source countries – and is either shipped to the UK in sea containers or simply sent via postal services.

“People sell from their Facebook pages and places like that to avoid detection,” says Gavin Terry, lead officer for intellectual property with the Chartered Trading Standards Institute. “Going back a few years, Ebay was the place to look, but as it has made their site more hostile for counterfeiters we’ve seen a displacement to social media and informalised selling channels.”

All the desirable brands get copied, he says, though he highlights MAC as one which he has seen aped.

Arsenic has been used by unscrupulous criminals, but is thankfully rare. Photograph: Alamy

That said, cosmetic scientist Colin Sanders thinks it is unlikely for arsenic to turn up in your cosmetics. “It would cost money to put it in,” he points out. Rat droppings are more of a possibility, although they are “not particularly harmful in small quantities,” says Sanders – so at least that’s reassuring.

Bargain-hungry shoppers may, however, fall foul of counterfeiters using the wrong amounts of normal ingredients, especially preservatives, which could trigger an allergic reaction in the unsuspecting. “Once you’ve developed an allergy to something you get it for a considerable period of time, possibly even life,” says Sanders. This means that even using the genuine product could prompt such reactions at a later date.

So how can you tell if your makeup is fake? The packaging is probably going to be a pretty good rip-off of the real thing in order to fool bargain hunters, Terry points out. The biggest tip, as ever, is to apply a bit of common sense: where are you buying your makeup from? Does the price seem feasible? “If it’s ridiculously cheap,” says Terry, “it’s either going to be counterfeit or stolen.”