Undercover footage revealing appalling animal welfare conditions at a horse abattoir has prompted the UK's Food Standards Agency to investigate.
The footage shows animals being beaten and neglected as well as alleged illegal procedures of slaughtering horses destined for European food markets.
Two slaughtermen seen in the footage have had their licences withdrawn, the FSA said.
The film was taken at Red Lion Abattoir near Nantwich, Cheshire, after an eight-week investigation by the Hillside Animal Sanctuary group.
It comes amid public anger that some of the biggest supermarkets have been selling beef burgers and other products that contained horse meat.
The abattoir footage shows horses being beaten with an iron rod to encourage them into pens. Some are crammed into the slaughter pens in pairs - on one occasion in a group of three - and are stunned together.
Under the Welfare of Animals Regulations 1995, horses should not be slaughtered in sight of one another because of the distress it causes.
Both the FSA and the RSPCA charity are investigating.
An FSA spokesman said after viewing the film the agency immediately withdrew the licences of the slaughtermen featured abusing animals in the footage.
'The FSA is also reviewing the footage and carrying out further investigations with a view to a potential prosecution,' he said.
Wendy Valentine, founder of Hillside Animal Sanctuary, said the groups has given a home to more than 200 horses in the past year.
'We just thought we would go and see what we were rescuing them from, so we decided to go and look at one of the country's biggest slaughterhouses,' he said.
'It was really traumatic. It was just so harrowing and horrifying.'
An RSPCA spokesman agreed the footage was upsetting to watch.
'We have concerns that horses appear to be in the stunning pen at the same time rather than individually, as the law requires,' he said.
'We also have concerns about some of the animals that appear to be injured. We have requested a copy of the unedited footage with a view to investigating.'
Red Lion Abattoir told Sky News it treats animal welfare and public health with paramount importance.
'In attendance at the the Red Lion Abattoir are three full-time food standards officers comprising of an official veterinarian and full-time meat hygiene inspectors throughout production,' it said in a statement.
The incidents in the footage are 'not the norm but of an isolated nature', it insisted.
The abattoir says it has taken disciplinary action against the individual featured.
'I agree horses should individually enter the stunning area and most certainly not three at a time,' it said. 'However, small horses and ponies having spent years together as companions are difficult to separate. Horse lovers would understand that.
'My opinion and that of other veterinarians is it is better to keep those types together to reduce the stress, providing swift dispatch is achieved.'
The Red Lion Abattoir said its meat was not part of the recent supermarket burger scandal.