British charity The Cure Parkinson's Trust has been condemned for funding "profoundly disturbing" experiments in which monkeys have their brains damaged to mimic symptoms of Parkinson's disease.
The suffering induced was said to be "severe" under the rating system used by the Home Office to assess animal tests.
In one case, the monkeys had already been subjected to similar tests in previous studies.
The research, published in 2011 and 2012, was conducted in Canada but was supported by the UK charity.
Both teams of scientists were testing ways to reduce the side effects of drug treatment for Parkinson's.
Marmoset monkeys were said to have endured "severe" suffering during the Parkinson's treatment experiments, co-funded by the Cure Parkinson's Trust
The experiments involved dosing marmoset monkeys with the toxic chemical MPTP to damage their brains and induce the symptoms.
High doses of the drug L-Dopa were then administered producing severe side effects including dyskinesia, or uncontrolled movement and psychosis.
The scientific papers about the experiments cite Cure Parkinson's Trust as a co-funder.
Andrew Tyler, director of the animal rights group Animal Aid, said: "It is clear that the vast majority of the British public do not want their money being used to fund profoundly disturbing experiments on animals of the sort co-funded by the Cure Parkinson's Trust.
"We are calling on charities like the Cure Parkinson's Trust to focus solely on productive non-animal research, which - unlike their terrible experiments on monkeys - can be directly applied to humans. It is only by using progressive non-animal techniques that we can hope to find a cure for diseases like Parkinson's."
My Tyler said the experiments conducted on the marmosets fell into the "severe" band of pain inflicted on the animal, the worst of three Home Office categories, which regulates UK animal testing.
He added that re-using animals in severe tests would not normally be allowed in the UK and Europe.
Mr Tyler said: "We've got the most severe category of suffering in which animals are being re-used paid for by the British public via a charity."
In a statement, the Cure Parkinson's Trust said: "In 2005, The Cure Parkinson's Trust, along with a wide variety of philanthropic institutions and individuals from around the globe, supported a programme of work which was under the direction of the University of Toronto and included some work looking at the role of serotonin in Parkinson's. It is to this work that we assume this condemnation is directed.
"The Cure Parkinson's Trust actually prefers to avoid the use of animals in its leading scientific research and indeed the scope of its work is almost entirely focused on clinical development in man using compounds with already proven safety data for other conditions and investigating their efficacy in people with Parkinson's.
"If The Cure Parkinson's Trust were ever to utilise preclinical models in its leading scientific research any work would adhere to the criteria set by the Association of Medical Research Charities guidelines.
"We have exciting progress to report later this week with new data from a trial funded by The Cure Parkinson's Trust on a Type 2 Diabetes drug called Exenatide."