The discovery of the first chemical to prevent the death of brain tissue in a neurodegenerative disease has been hailed as the "turning point" in the fight against Alzheimer's disease.
More work is needed to develop a drug that could be taken by patients.
But scientists say a resulting medicine could treat Alzheimer's, Parkinson's, Huntington's and other diseases.
In tests on mice, the Medical Research Council showed all brain cell death from prion disease could be prevented.
Prof Roger Morris, from King's College London, said: "This finding, I suspect, will be judged by history as a turning point in the search for medicines to control and prevent Alzheimer's disease."
He told the BBC a cure for Alzheimer's was not imminent but: "I'm very excited, it's the first proof in any living animal that you can delay neurodegeneration.
"The world won't change tomorrow, but this is a landmark study."'