Up to a quarter of a million adults with mental illnesses are not being supported by an appropriate adult (AA) while in police custody, according to a report.
An analysis of police data showed appropriate adults - trained volunteers who help with communication, welfare and legal rights - were used in around 45,000 of the 1.4 million detentions and voluntary interviews of adults each year, even though up to 280,000 involved those who are mentally vulnerable.
Home Secretary Theresa May, who commissioned the report by the National Appropriate Adult Network, said the situation was "not acceptable" and she would review the charity's recommendations.
Appropriate adult schemes were introduced in the 1980s to repent miscarriages of justice involving vulnerable people and local authorities have a legal duty to provide them for children.
But the report, entitled There to Help, said some areas had no organised schemes for vulnerable adults.
And police custody sergeants surveyed by the charity reported difficulties in getting appropriate adults during weekends, evenings and overnight.
Of those who completed the survey, around a third said they received no training in identifying vulnerable suspects, while some reported spending hours trying to find a suitable AA, admitting to sometimes asking random members of the public or proceeding without one.
Mrs May said: "Appropriate adults provide vital support and help to de-mystify what can be a confusing, sometimes frightening, experience in police custody.
"Evidence suggests there is a lack of appropriate adults to safeguard the welfare and rights of mentally vulnerable adults in police custody. That is why I commissioned this review to determine where the problems lie.
"The status quo is not acceptable and I am concerned that vulnerable adults are not always receiving the support of an appropriate adult. We are currently examining the recommendations and implementation options to ensure that vulnerable people are provided with the support they are entitled to."
Chris Bath, chief executive of the National Appropriate Adult Network, the charity which led the study said: "People with learning disabilities, mental ill health, traumatic brain injuries or autistic spectrum disorders are some of the most vulnerable citizens, and state detention is perhaps the most vulnerable situation. We have a moral and a legal duty to ensure appropriate adults are available wherever people live."
The recommendations in the report include a national framework for the provision of appropriate adults, a statutory duty on police officers to secure one and improvements to police training and record keeping on vulnerable adults.