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."