A British charity 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 supported by Cure Parkinson's Trust, a UK charity founded in 2005 by four sufferers of the disease.
Both teams of scientists were testing novel ways to reduce the side effects of drug treatment for Parkinson's.
The experiments involved dosing marmoset monkeys with the toxic chemical MPTP to damage their brains and induce Parkinson-like symptoms.
High doses of the Parkinson's drug L-Dopa were then administered producing severe side effects including dyskinesia, or uncontrolled movement, and psychosis.
Each of the scientific papers, published in the Journal of Neuroscience and Public Library of Science ONE, cites Cure Parkinson's Trust as a co-funder.
Andrew Tyler, director of the animal rights organisation Animal Aid, which highlights the experiments on its website, 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."
The Home Office, which regulates animal tests in the UK, categorises degrees of pain, distress or lasting harm caused to animals as "mild", "moderate" and "severe". Only a small minority of laboratory animals fall into the "severe" category in the UK.
The experiments conducted on the marmosets fell squarely into the "severe" band, said Mr Tyler.
He added that re-using animals in severe tests would not normally be allowed in the UK and Europe.
A passage from the first paper, describing an experiment in which monkeys were treated with a form of ecstasy, reads: "The animals used in the current study were not drug naive and had been used in previous studies assessing the anti-dyskinetic potential of adjunct therapies."
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."
No-one at Cure Parkinson's Trust was available for comment.
The Cure Parkinson's Trust said in a statement: "The potential for the use of Ecstasy as a treatment for Parkinson's was first brought into the public eye in early 2001 in a documentary entitled The Agony And The Ecstasy which followed the fortunes of a young man with Parkinson's who found that taking Ecstasy completely normalised his Parkinson's symptoms. After this there was a flurry of work in the area right around the world.
"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."