Some police forces are being "overwhelmed" and are unable to respond to urgent calls on time, the police watchdog has revealed.
An examination of all 43 police forces across England and Wales by HM Inspectorate of Constabulary has highlighted "major concern that policing is under significant stress".
The inspectorate said that, 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.
The annual assessment of police effectiveness found thousands of emergency calls were being held in queues, largely because officers were not able to respond to them.
The watchdog said it was "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.
And 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.
The inspectorate said earlier concerns about a significant shortfall in the number of detectives were being addressed, but the process is slow.
Currently, one-in-five detective desks is 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 and significant pressures on their budgets.
Overall one force, Durham Constabulary was rated "outstanding"
30, including West Yorkshire and Thames Valley have been given a "good" rating.
But 12, including the three biggest forces, Greater Manchester, West Midlands and the Metropolitan police force "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."
Peter Kirkham, a former Detective Chief Inspector with the Metropolitan Police told Sky News that 25% budget cuts over recent years were now having "dangerous consequences".
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He added: "Chief officers have got impossible decisions every day and frankly it is like rearranging the deckchairs on the Titanic.
"They tend to take resources to where they've been criticised, sort that out. Then in that area where those resources were taken, that place starts to flounder."