Crimes are being investigated by inexperienced uniformed officers because of a shortage of detectives, the police watchdog has said.
A report found offences like burglary and theft are often resolved over the phone or assigned to "beat" officers.
HM Inspectorate of Constabulary said a quarter of victims are not receiving the service they should as a result .
It said police in England and Wales are delivering a good level of service with "dwindling resources".
But it also warned that "cracks in the system" are widening.
The report looked at a representative group of 14 constabularies in England and Wales including the West Midlands and Greater Manchester forces.
Failings mentioned in the report include evidence not being gathered and potential lines of inquiry being missed.
Risk being 'irrelevant'
HM Inspector of Constabulary Matt Parr said: "There's a shortage of detectives to do the routine detective work, and very often it's being farmed out to people who do their best but are not trained at the same level."
Last year, a House of Commons report warned policing risked becoming "irrelevant", with vanishing neighbourhood presences and low detection rates.
A Home Office spokeswoman said: "We recognise new demands are putting pressure on the police and we are committed to ensuring they have the resources they need.
"This is why we have provided more than a £1bn increase in police funding compared to last year, including council tax and funding to tackle serious violence.
"There are clear areas for improvement for some forces and we expect them to consider these findings carefully and to identify and implement improvements swiftly."