More than two thirds of police forces in England and Wales are letting down vulnerable victims, a new report suggests.
Four services were rated as "inadequate" for how they protect and support those at risk because of factors such as a disability, their age or having been subjected to repeated offences.
Another 27 had shortcomings in at least one of the areas assessed by watchdogs, who warned that "small failures may have tragic consequences".
Inspectors highlighted concerns about units which specialise in protecting vulnerable people being overstretched.
They found teams are understaffed due to large numbers of vacant posts, maternity leave or long-term sickness absence, expert officers are being diverted onto other duties and some forces rely on "on call" personnel at weekends or overnight.
The findings came as a linked inquiry revealed that units dedicated to domestic abuse are are on the verge of being "overwhelmed" following a "staggering" 31% surge in reported cases.
Inspectors examined how forces respond to and safeguard those who are vulnerable in some way, with a focus on missing and absent children, victims of domestic abuse and how well prepared they are to tackle child sexual exploitation (CSE).
A report published today by Her Majesty's Inspectorate of Constabulary (HMIC) found that:
:: There is a lack of high quality data across the police service in relation to vulnerable people, with eight forces not collecting information at all.
:: Risk assessments are not conducted in some cases because staff are given "discretion" over whether they should be completed.
:: Processes to identify children at risk in households where domestic abuse takes place are not reliable or effective.
:: Victims are not being provided with updates on cases.
:: Variations in how forces define and collect data on missing and absent children mean some do not have a good understanding of the nature and scale of the problem in their area.
:: Implementation of measures to tackle CSE is "patchy".
Inspectors collected data, reviewed case files, observed meetings and heard from domestic abuse victims before grading forces on their effectiveness at protecting vulnerable people.
No force was "outstanding", while 12 were given a rating of "good", with the remaining 31 either having "causes for concern", "areas for improvement" or both.
Bedfordshire, Essex, Staffordshire and Surrey constabularies were judged to be "inadequate" - meaning there were "serious weaknesses" in their arrangements to safeguard and investigate cases involving vulnerable people.
The report said: "The number of forces judged to be inadequate or to require improvement is high.
"We would stress that ... many forces provide a good service to some vulnerable victims, some of the time; but there are important areas in which the police response needs to improve, and in which small failures may have tragic consequences."
Mark Castle, chief executive of charity Victim Support, said: "This report makes disturbing reading, highlighting widespread failure by the police to identify, assess or support the most vulnerable victims of crime, in particular children."
An NSPCC spokesman said it is "simply not good enough" that police are "failing to properly investigate child sexual abuse and exploitation".
Zoe Billingham, who led the investigations, said: "Victims of crime and anti-social behaviour who are vulnerable in some way are in greatest need of the police's protection and support, but HMIC's inspection found that most forces require improvement in how they protect and support them."
She added that there was a "clear commitment" to improve services and many examples of officers and staff "going the extra mile".