Dozens of frozen brains kept for research on conditions including mad cow disease, Parkinson's disease and Alzheimer's melted after a freezer broke down.
The organs were preserved at -80C but were found rotting in a pool of blood and ice at St Bernard's Hospital in Ealing, west London.
The mistake was blasted in a secret report into the incident obtained under the Freedom of Information Act.
The report said many hospital managers were not aware that the collection existed and many of the donated brains were covered in dust, with some specimens "lying open with spiders crawling over them".
The meltdown was blamed on the fact the freezer alarm had not been routed to the new extension of the doctor in charge.
Prof Thomas Barnes, report author, said that the cleaning and maintenance of the collection was "not fit for purpose".
"If the collection is to continue to be maintained, then work needs to be done to ensure the collection and staff who work in it are safe and that it can be accessed in a way which is professional and efficient," the Daily Star quoted him as saying.
The West London Mental Health Trust is responsible for the brains.
A spokesman for the Trust said: "The freezer is checked at least once a month and it was during such a routine check that the fault was found.
"The freezer alarm system has -subsequently been addressed to help prevent a similar incident occurring.
"The Trust has been responsible for the collection for more than 12 years without prior incident," he added.