Cambridge University is investigating reports that sheep used in medical research at its laboratories experienced distressing levels of mistreatment and suffering.
Animals used to study neurological disorders had electrodes implanted into their brains and were left disorientated, struggling to walk and losing their sight before they died or were killed.
The experiments are pointless because of the differences between sheep and humans, according to the British Union for the Abolition of Vivisection (BUAV).
Undercover video footage from the BUAV also exposed a number of incidents that were said to have increased the misery still further.
They included one animal suffering a broken leg and being put to sleep after an impatient staff member tried to force her into a cage for weighing.
Another sheep named Jane had to be killed because she had a faulty implant, while a third animal, Janet, suffered severe weight loss and became virtually blind.
She was allegedly left in this state for several days, lying in her own faeces, before being euthanased.
The purpose of the experiments was to investigate the early stages of Batten's Disease and Huntington's Disease, two serious neurological conditions for which there is no cure.
Animal research is regulated by the Home Office, which applies rules intended to ensure that it is only carried out when necessary and suffering is kept to a minimum.
Dr Katy Taylor, the BUAV's head of science, said: "It is clear from our investigation that these poor animals suffered a great deal and, we believe, unnecessarily. This shows why undercover investigations are essential to reveal the extent of cruelty and suffering animals endure in UK research laboratories, often for trivial purposes or highly speculative science.
"There are serious questions about whether the research has any realistic prospect of advancing the search for cures for these diseases and about the way in which the Home Office has applied the harm/benefit test required under UK legislation before allowing such research to go ahead."
A statement from Cambridge University said sheep were used in the studies because they had "complex brains" similar in size to a large monkey's, and no alternative existed.
The statement added: "The researchers have been testing a sheep model of Huntington's Disease developed by collaborators in New Zealand and Australia and studying a line of sheep that carries a natural mutation for Batten's Disease.
"Whilst every attempt is made to keep distress to a minimum, the very nature of these diseases means that the animals will show symptoms related to damage of the nervous system similar to those seen in humans..
"We take the allegations of mistreatment of animals very seriously and will take all appropriate actions necessary to investigate this matter in accordance with the university's responsibilities under our Home Office licence and our own Animal Welfare Ethical Review Committee."