West Yorkshire Police have been storing DNA samples from suspects and car crash victims in a freezer also used for ice cream, a watchdogs' report has said.
The criticism comes in a report from the prisons watchdog and the chief inspector of constabulary.
The inspectors said the management of the samples needed urgent attention to avoid undermining prosecutions.
West Yorkshire Police said the samples found in the freezer were of "no value" and would not have been analysed.
The inspectors also said staff at police custody suites had been confused about which sample belonged to which suspect.
West Yorkshire Police have 13 blocks of cells legally approved to hold suspects following their arrest.
Inspectors from the two watchdogs visited a sample of these cells and accompanying facilities in Leeds and Bradford in October 2008.
The watchdogs said they were concerned about how officers were handling DNA, blood and urine samples taken in the custody suites, many of which would be used as evidence in prosecutions.
"Samples were incorrectly stored in fridges and freezers alongside ice cream, with some improperly bagged," said the report.
"This led to confusion among staff tasked with submitting samples, so many were not submitted for analysis and had been allowed to remain in freezers for a number of years."
One sample, labelled as relating to someone killed in a traffic accident, had neither been sent for toxicology tests or disposed of.
"We were unsure whether it really related to a road death investigation or whether there was an error on the label," the report said.
The watchdogs said that some freezers were insecure and some samples had defrosted.
"The force was potentially missing opportunities to bring offenders to justice and solve old cases," said the inspectors.
"These practices were exposing the force to unacceptable levels of risk. The maintenance of public confidence in forensic evidence is crucial."
A spokeswoman for West Yorkshire Police said: ""The report refers to one instance of ice cream being found in a fridge near forensic samples.
"These samples were hair, blood, urine and fingernail samples from people who had been eliminated from criminal enquiries.