A leading producer of foie gras has been using engine oil to grease the pipes it uses to force down birds’ throats.

Secret footage taken by an undercover investigator shows oil intended for vehicles being applied to the rods before they are shoved into the geese to force-feed them.

The clips, taken on a farm in Ukraine, also show birds being thrown violently into cages, leaving them injured and in pain.

Injured and dead birds were left to suffer or rot in piles, the witness reported.

Foie gras is the fatty livers of geese force-fed to make them unnaturally overweight. Its production is considered so cruel that it is banned in the UK but imports are legal. Campaigners have been lobbying for an import ban after Brexit.

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The investigator who used a hidden camera to film the footage said he saw geese clearly suffering after being force-fed then tossed aside. Some vomited, some panted.

They had already suffered from being squashed together on lorries, he said.

“What I saw cannot be compared with what I originally expected to see. I worked there for about a month so as not to cause suspicion by leaving after the first day, but it was very difficult for me.

“They bring birds for breeding in a truck. In each section there should be 12 birds but in fact there are about 40, therefore they are already in terrible stress and aggressive to each other.

“Birds are thrown from the height of the truck into cages that are also very small.

“I saw a lot of cases where birds were trying to get out of the cage but were clinging to the pintle [hinge bolt] and immediately died directly on them or from wounds.”

Force-feeding, which makes livers swell to 10 times their normal size, also often causes internal injuries.

Ukraine has only one foie gras producer, MHP, which is the country’s largest poultry producer. Its keeps 20,000 birds in an industrial-style foie gras unit.

Last year the company said it sold 50,000 tons of foie gras, mostly exported.

The country’s animal-welfare laws ban painful and injurious methods to obtain animal products but the witness said about 20 birds a day died from force-feeding or injuries.

It is not known whether Ukrainian foie gras ends up on sale in Britain but France, the Netherlands and China all import it.

Any foie gras sold in stores in the UK came from farms with similar levels of animal cruelty Connor Jackson

Fortnum & Mason and Harrods in London both sell geese livers but it is not known whether they source foie gras from Ukraine.

Animal-welfare group Open Cages, which obtained the footage, said the violent force-feeding, violent handling and injuries happened on farms that supply Britain.

“The practices documented here have been seen time and time again on foie gras farms, meaning that any foie gras sold in stores in the UK came from farms with similar levels of animal cruelty,” said chief executive Connor Jackson.

“By allowing the sale of foie gras on our shores we are putting money in the pockets of this disgraceful industry.

“While imported foie gras remains on the menu, these poor animals’ cries will haunt us for years to come.”

A spokeswoman for MHP said the company was “deeply concerned about the practices that appear to be shown in this video”, adding: ”As a result, we are undertaking a full investigation and audit of the facility.

“We take our responsibilities to the animals we rear very seriously. MHP’s policy on the humane treatment of animals has been created in line with global best practice and covers the process from production to shipment with an in-built cycle of continuous review and improvement.

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“As a certified exporter to the EU our facilities are regularly audited by the relevant authorities.

“If our high standards have not been met, we will take immediate action to correct this. We will not compromise on animal welfare.”