The upsetting video footage shows puppies and kittens panicking as they are injected with needles before being dissected in a UK animal testing lab.
The film was taken by a brave undercover investigator, who worked at the centre for eight months until December last year while reporting back to the British Union for the Abolition of Vivisection (BUAV).
Known only as Susie, the undercover student filmed playful young animals screaming and fretting after staff took them from their mothers.
Their healthy mothers were also killed, almost always without any apparent attempt to find them loving homes.
Susie witnessed scenes she never again wants to see - but the BUAV would like them to be brought to the attention of the public, in a bid to make them realise the full extent of animal testing procedure.
Susie documented 92 beagle puppies, 10 adult nursing female beagles, at least 15 kittens and an unknown number of rabbits, calves and chickens being killed.
She told the Sunday Express: “I found it terrible because I knew there was nothing I could do to help them.”
During her time at the lab, Susie managed to save two adult dogs who were due to be killed, Bonnie and Billie, and a five-month-old puppy, Oliver.
BUAV’s spokeswoman Sarah Kite said: “People across the country will be deeply upset to learn what is happening to puppies and kittens inside the secret world of animal research.
"It is unacceptable not only that these animals are suffering and dying in this way, but that many of them could have been released into loving homes instead of being killed and discarded for convenience sake.
“We do not believe that the company or the wider research community are doing enough to use alternative approaches to animal testing in the development of animal vaccines.”
The centre uses the animals in tests for veterinary vaccines for diseases such as kennel cough, parvovirus and feline calicivirus. MSD says it is obliged by law to test the drugs on the young creatures.
A spokesman for Merck said: “MSD Animal Health adheres to all regulatory standards of testing and development of vaccines and our standards for animal care meet or exceed applicable local, national and international laws and regulations.
“All MSD animal health research is performed by qualified, trained personnel. Our facilities are in full compliance with all laws and regulations, and procedures and facilities are regularly reviewed and inspected by relevant regulatory authorities."
“We are dedicated to the ethical treatment of all animals used in development of medicines and vaccines. “Measures are taken to assure that the fewest numbers of animals are used and that any discomfort is minimised.
“Whenever possible, MSD Animal Health makes significant efforts to find homes for the animals.”
For more details about the BUAV investigation and to see the full video, visit buav.org.