The Police Complaints Authority (PCA) said its research found that untrained officers left in charge of station cells were holding people illegally and failing to give them medical attention when they need it.
Better training could help cut the number of deaths in custody, the report added.
![[ image: width=150]](/olmedia/90000/images/_90570_police_officer_cautioning_prisoner_in_cells150(10-05-98)bbc.jpg)
Custody sergeants are legally responsible for ensuring prisoners are treated in accordance with the Police and Criminal Evidence Act.
The legislation covers the length of time a suspect can be detained and guarantees a prisoner's rights to see a solicitor and receive medical attention.
But in the PCA's survey of 620 custody officers across all 43 police forces in England and Wales, 23% had not attended a custody officer training course.
Only seven officers had been given refresher training during their careers.
The authority said it had "particular concern" about some forces which allow officers to work as custody sergeants for years without proper training.
Many forces appeared to assume an experienced sergeant would automatically know enough to carry out the role, the PCA said. But the research had found custody officers authorising detention when it was unlawful.
Others failed to contact a doctor when prisoners were mentally ill or had taken a "life threatening cocktail of substances".
![[ image: width=150]](/olmedia/90000/images/_90570_generic_police_station_sign150(10-05-98)bbc.jpg)
The PCA, which independently scrutinises the police, said every officer should receive full training in the PACE legislation and first aid before they become a custody sergeant.
It added that they should also be trained in food and hygiene and awareness of suicide risks and drug and alcohol abuse.
"On average every police force will experience one death in custody each year," said the PCA report.
"Some forces, both large and small, experience a disproportionate number. Some (though not all) of these deaths could be avoided if the police surgeon and custody sergeant were picking up the indicators of risk and if they had an effective communication system enabling appropriate and timely action to be taken."
The Home Office: Administrative burden on police forces
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