Footage allegedly showing sheep being killed when not stunned properly and being dragged by their legs and throat has been obtained by an animal welfare charity.
Animal Aid said investigators got four sets of films from visits to Farmers Fresh abattoir in Cross Lanes, Wrexham.
It has now called on the Welsh Government to make the use of CCTV in abattoirs mandatory.
A criminal investigation is under way. The company has made no comment.
Animal Aid’s Tor Bailey said: “Without our covert cameras in place, this horrendous situation may have continued unchecked for some time.
“We’re calling on the Welsh Government to introduce independently monitored mandatory CCTV without delay across Wales.”
CCTV is not compulsory in Welsh slaughterhouses and the charity said it should be brought in as a “necessary safeguard”.
A Welsh Government spokesman said: “We take animal welfare at slaughterhouses very seriously.
“As a criminal investigation is under way, we are unable to comment further at this stage.”
The Food Standards Agency – which is carrying out the investigation – said: “We took swift action to increase our presence at Farmer Fresh in Wrexham and to introduce additional checks and ensure animal welfare was being protected.”
A Farmers Fresh spokesman said the company had “no comment” to make.