Young birds were seen flailing in distress on their backs at the UK’s largest supplier of ducks to leading supermarkets including Sainsbury’s, Tesco and Morrisons, according to undercover activists.

The ducklings, also supplied to Chinese takeaways, were filmed struggling on their backs to right themselves in footage taken by the campaigners.

Birds were seen in the video being slung onto the floor “like trash” after having their necks broken.

Gressingham Foods, which owns the farms, strongly denied claims the fowl was being mistreated, saying: “There is no evidence of abuse or neglect by Gressingham staff in this footage.”

But a spokesperson added: “There are a limited number of images and comments that are not representative of the high standards expected of our farms.”

The undercover footage, taken at two farms in Norfolk endorsed by the Red Tractor scheme, also appeared to show the birds had been deprived of open water to swim in.

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As many ducks and geese are killed in Britain each year as turkeys – about 15 million, according to government figures – many for the Christmas market. Activists said they were subjected to a “miserable existence”.

The witnesses, from the Animal Justice Project, claimed they also saw:

Ducklings on their backs crying out and “clearly in great distress”

Animals with a 5ft wingspan living in crowded sheds with no water to swim in

One shed housing 6,500 ducks had just 50 drinking stations – serving 130 birds each

Some birds appeared to struggle to support their own weight on sagging legs

Workers did not wear gloves or overalls, just wellington boots

Lame birds which appeared to have been seen by workers were ignored, the footage suggested.

The floor was “sodden from ducks throwing water over themselves to preen”, activists added.

The video also shows some workers grabbing ducks by their necks, carrying them through the sheds then breaking their necks and throwing them onto the flock.

The more the birds pedalled, the more exhausted they became, witnesses said (Animal Justice Project)

Gressingham, which each year rears in barns up to 8 million ducks – a Pekin cross mallard – also strongly denied the footage showed evidence of systemic abuse or neglect, or of birds being left to die in pain, saying its farms were subject to full independent audit.

According to Animal Justice Project, the longer the ducks were on their backs, the harder it became for them to right themselves.

Activists said the footage showed a failing by the industry, as by law animals must have the ability to behave naturally for their species.

“Ducks are waterfowl, have evolved to be in water, and access to open water has been shown to be essential for improving and maintaining the health, plumage condition and cleanliness of Pekin ducks,” said Claire Palmer, a spokeswoman.

“The duck industry has not improved over the years, it has got worse. Scenes of flailing and filthy birds on their backs are difficult to watch but are the sad reality for many farmed ducks. We don’t need to be subjecting these birds to a life of misery.”

Alick Simmons, a former UK deputy chief veterinary officer, said of birds that were lame or on their backs: “I am confident that the birds are likely to be in some distress and that treatment and/or euthanasia should be administered as soon as possible.”

The apparent lack of open water contributed to an “unstimulating, barren environment”, he said.

In a statement, Gressingham said: “Any inexperienced, untrained intruders entering our farms will cause distress to the birds and they may demonstrate unnatural behaviour.

“The welfare of our birds is an absolute priority. We work to the highest standards in hygiene, biosecurity and animal welfare across all our farms. All Gressingham Foods farms are subject to regular internal and external audits and are under veterinary supervision.

“All staff responsible for the welfare of our birds have the necessary training with regular updates for continuous improvement.

“Our ducks are reared free to roam in large barns with natural light, fresh air, access to water for preening, access to fresh drinking water and feed throughout the day. We also apply a fresh bedding of straw every day.”

A spokeswoman added: “There is no evidence of birds being left to die in pain after culling of birds which are failing to thrive or appearing lame.

“The footage shows some individual birds being carried after neck dislocation. The flapping activity described is the normal involuntary reflex activity after death by neck dislocation, and such birds are unconscious, insensible to pain and are not suffering.”

“The footage does indeed show some individuals with walking difficulty and this can be a feature of individual birds in all flocks. This is under regular review by our veterinary consultants but there is no evidence this is specifically caused by wet floors or unusual growth rates.”

Lame birds that cannot hold their own body weight are humanely culled, she said.

“The footage shows some individual ducks that have been cast on their backs. This can occur when a flock is disturbed during the rest period, especially by untrained intruders, when smaller individuals may be bowled over by the general activity of a healthy flock.”

Andrew Opie, of the British Retail Consortium, said: “Our members take their responsibilities to animal welfare very seriously. The BRC led the calls for the introduction of unannounced visits to farms across the UK to ensure compliance with all welfare standards. Any breaches to animal welfare are totally unacceptable, and will be investigated immediately, with swift action taken to rectify any issues discovered.”

A spokesperson for Red Tractor said: “Protecting animal health and welfare is one of our top priorities and we take any allegations of breaches to our Standards very seriously. As soon as we were made aware of the footage, we launched an immediate investigation to substantiate the claims and to review the behaviours seen. Corrective action will be taken where necessary.”

A Sainsbury’s spokeswoman said: “The welfare of our animals is extremely important. We’re concerned by this report and urgently investigating with Gressingham.”

A Tesco spokesman said: “We require high animal welfare standards from all brands sold at Tesco, so these claims are deeply concerning. We will be urgently investigating this with them.”

The Independent has also asked Morrisons to comment.

The Co-op said Gressingham did not supply any of its own-brand duck and was looking into whether it sells other-brand Gressingham duck.