The report is the latest to identify problems with handling DNA material
Failings in the way police officers in London dealt with DNA samples linked to violent crime, rape and murder have been highlighted in a report.
The inspection found samples had been left in a freezer at two police stations in Hackney, east London, instead of being sent for analysis.
The findings came in a joint report by HM Chief Inspectors of Constabulary and of Prisons.
Scotland Yard said it was satisfied no investigation had been compromised.
The report found some DNA samples held in a freezer and fridge in a CID office at Shoreditch police station had not been submitted to the National DNA Database, where they could be matched to other DNA profiles.
In Stoke Newington, a police freezer contained DNA samples dating back almost a year.
The investigation was by Chief Inspector of Constabulary Denis O'Connor and Chief Inspector of Prisons Dame Anne Owers.
The Metropolitan Police said procedures for handling DNA had been reviewed.
A spokesman said: "In what was an overall positive report into custody suites on Hackney borough, the HMIC (HM Inspectorate of Constabulary) raised some concerns about the retention of a small number of DNA samples.
"The HMIC did not make any specific recommendations, and we are satisfied that no investigation was compromised.
"However, Hackney borough acknowledge the comments and has, as a result, reviewed its supervision of the DNA retention process."
BBC home affairs correspondent Danny Shaw says the report is the latest in a series identifying problems in the handling of forensic material by police.
Last year, inspections of custody suites in West Yorkshire and Cambridgeshire found specimens had been stored alongside ice cream and raw meat.