Police are taking days to respond to emergency calls that should be acted on within an hour, a watchdog has revealed.
In some cases, victims of violence and other serious crimes face long delays as forces are unable to dispatch officers promptly.
Inspectors found instances where 999 calls that were graded as needing a "prompt" response - meaning they require action within 60 minutes - were left unattended for several hours or even days.
Her Majesty's Inspectorate of Constabulary and Fire & Rescue Services concluded that around a quarter of forces in England and Wales are "all too often overwhelmed by the demand they face".
The watchdog's annual assessment of police effectiveness found thousands of emergency calls are being held in queues, largely because officers were not able to respond to them.
Inspectors came across occasions where police could not respond to victims at all, or did so badly, the report said.
It added: "HMICFRS is concerned by this finding, because it shows that the system is under severe strain and in some forces the cracks are showing."
Inspectors said "life and limb" and "crime in action" cases were generally dealt with quickly.
But responses were sometimes delayed for 999 calls in the next most serious category - such as where someone has been assaulted but the offender is no longer on the scene.
In two forces there were "considerable delays" in allocating calls for assistance, while in one area between 20% and 50% of incidents to which a unit should have been sent within 24 hours did not meet the target, according to the report.
HM Inspector Zoe Billingham said: "About a quarter of forces are all too often overwhelmed by the demand they face, resulting in worrying backlogs of emergency jobs.
"We can see people waiting a long, long time for that 999 response and our concern here, in particular, is where there are vulnerable victims in that backlog."
The inspectorate's review also detailed how:
- A large number of crimes are effectively written off, denying victims the service they are entitled to.
- In some cases, basic tasks such as carrying out house-to-house inquiries to search for witnesses or checking for CCTV were overlooked at crime scenes.
- Forces are failing to track wanted suspects and develop intelligence to identify their whereabouts.
- As of July last year, there were 60,061 "wanted records" on the police national computer - including 374 relating to homicide and 1,135 to rape.
- Police have yet to assess the risk posed by 3,300 registered sex offenders.
- Forces are not making routine use of intelligence databases to carry out background checks when they arrest foreign nationals.
- The use of police bail has reduced by three quarters (75%) after curbs were introduced to stop people languishing in legal limbo for months or years.
- One in five detective desks are empty or filled with unqualified staff amid a national shortfall of 5,000 investigators.
HMICFRS emphasised that most forces are continuing to do a good job keeping the public safe in the face of "dramatic" increases in demand.
The report noted a number of areas of good work, including investment in digital forensics, co-operation with other agencies on serious and organised crime and increases in the number of officers assigned to neighbourhood policing.
Ms Billingham said: "I congratulate officers and staff on the way they have largely kept policing standards high."
Overall one force was rated "outstanding", 30 were "good" and 12 "require improvement".
Sara Thornton, chair of the National Police Chiefs' Council, said: "HMICFRS finds good service overall and improvements since last year's inspection, but it is also evident that increases in demand are impacting on forces' ability to meet standards in some important areas.
"In the last year, policing has been under real strain with rising crime and demand that is more complex, an unprecedented terror threat, and officer numbers at 1985 levels.
"We talked last September about how the response to terror attacks had led to backlogs of incidents and a slower response to the public."
Labour MP Yvette Cooper, chairwoman of the Commons Home Affairs Committee, said: "This report shows a deeply serious strain on our emergency service.
"For 999 emergencies to go without a response for hours or even days is shocking, and deeply distressing for victims and undermines public safety and the fight against crime."