Almost 500 body parts have been stored by police in England, Wales and Northern Ireland since 1960 in cases where inquiries have closed, a report has found.
Police forces are able to keep samples after post-mortem exams if there is a "legal requirement" to do so, but a report by the Association of Chief Police Officers (Acpo) found no record as to why 492 samples were kept.
Force chiefs apologised to grieving families as Acpo revealed that investigators "may have wrongly assumed that the human tissue seized at the post-mortem examination had been disposed of by the medical profession or by some other means".
Deputy Chief Constable Debbie Simpson, who led the audit, said there was "no nationally agreed policy to deal with such items at the conclusion of the investigation".
Some relatives may not have been made aware that detectives had kept the remains but officers are now in the process of "sensitively" dealing with the human tissue, she added.
The Northern Ireland Police Service retained 71 samples, West Midlands police kept 40, while Scotland Yard held 39.
Thirteen forces reportedly said that they had not held any samples, and Scotland was not included in the audit.
According to Acpo the samples - which included complete internal organs, brains and human limbs - were stored "until convicted prisoners have served their sentences".
The report said police were currently "dealing" with the samples, and would tell family members where it was appropriate.
The report did not look at cases that were still under investigation, or could be appealed.
Under the Police and Criminal Evidence Act investigators can retain body parts to help in investigations.
But in 2006 new legislation made it illegal to retain body tissue without informing relatives.
Acpo launched its audit following cases that emerged earlier this year in which families were not told that body parts and samples had been kept.
In May, it was discovered that police in Hampshire and the City of London kept 89 samples in cases of suspicious and unexplained deaths without telling relatives.
One woman in Southampton was not told that police had kept the brain of her six-year-old son in a jar for more than 12 years.
She only found out when police knocked on her door and told her.
"I'm gobsmacked - it's shocked me that it's that much," the woman told the BBC.
"Not that you have gone through enough back then, it's horrendous.