Fully-fledged police officers and detectives catch more crooks than civilian staff, according to research.
The Police Federation of England and Wales said there is a direct link between officer and detective numbers and crimes solved.
An analysis of Government figures revealed that forces which have maintained a high ratio of officers to civilian staff have the best detection rates.
These include Cleveland, Merseyside, Northumbria and Lancashire, which have all registered improved crime-solving figures over the past decade.
But those which have reduced the number of officers relative to civilian roles, in some cases to almost equal numbers, are faring badly.
These include Surrey, Northamptonshire, Suffolk, Lincolnshire and the West Midlands, where the proportion of criminals brought to justice has fallen.
North Wales Detective Inspector Steve Williams, chairman of the Police Federation National Detectives Forum, said the figures are "quite shocking".
He said: "The number of cops, and in particular detectives, makes a huge difference to the detection of crimes being committed.
"Neglecting detectives will lead to an increase in the fear of crime. I would argue that as detection rates go down, so does public satisfaction.
"If anything happened to you or your family, you would want an experienced, trained and fully-fledged detective."