Shocking undercover footage which reveals appalling animal welfare conditions at a horse abattoir has prompted the 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Two slaughtermen seen in the footage have had their licences withdrawn, the FSA said.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 beefburgers 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 and on one occasion in a group of three, and are then stunned together.
Horses are also hit with sticks to goad them into slaughter pens.
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 it was made aware of the footage which it says shows animal welfare offences carried out by slaughtermen in the abattoir.
"The FSA believes it is important that action is taken against people who commit animal welfare offences. So, after viewing the film the agency immediately withdrew the licences of the slaughtermen featured abusing animals in the footage. This means they cannot continue to slaughter animals," the spokesman said.
"The FSA is also reviewing the footage and carrying out further investigations with a view to a potential prosecution."
The most serious concerns identified in the footage are several occasions of more than one horse in the stun box at the same time; excessive use of a stick on horses; hitting horses with a rope, the FSA said.
Craig Kirby, head of approvals and veterinary advice at the FSA, said: "I was shocked by the footage and that is why we took immediate action to make sure the individuals involved could not continue to slaughter animals. The FSA takes animal welfare at slaughter very seriously, and we will always investigate and take action when we see breaches of the legislation."
Wendy Valentine, founder of Hillside Animal Sanctuary, said: "We have given a home to more than 200 horses in the last year and we have more than 700 horses all together.
"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.
"It was really traumatic. It was just so harrowing and horrifying."
An RSPCA spokesman said: "The footage is shocking and upsetting to watch. With the Welfare of Animals (Slaughter or Killing) Regulations 1995 in mind, we have concerns that horses appear to be in the stunning pen at the same time rather than individually, as the law requires.
"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 that 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. 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 also insisted that its meat was not part of the recent supermarket burger scandal.