Around a third of the police workforce in England and Wales is not classed as being on the "front line", according to Her Majesty's Inspector of Constabulary.
It disclosed the proportion in a report after asking members of the public and police representatives what they thought constituted frontline work.
Of all staff in the 43 police forces, 68% were on the front line. About 61% of both police officers and police community support officers (PCSOs) work in visible frontline roles such as emergency response.
The proportion available at any one time varied, with an average 12% of officers and PCSOs in visible posts and available to the public at key times.
But Sir Denis O'Connor, the chief inspector of constabulary, said maintaining some forces' levels of frontline coverage would be hard. He said: "In a nutshell it looks difficult for the front line to remain in its current form in a number of forces."
The study, the first to come up with a definition of "front line", will be pored over by Home Office bosses overseeing cuts to policing in England and Wales. The inspectorate said front line means "those in everyday contact with the public and who directly intervene to keep people safe and enforce the law".
Front line involves jobs such as CID and asset confiscation, as well as fingerprint and scenes-of-crime jobs. Middle office and back office posts comprise tasks such as process support, training and IT and communications.
Sir Denis, a former chief constable of Surrey Police, stressed that staff in middle or back-office posts were key and ensured frontline duties were properly carried out, so making cuts presented a "big challenge". He added: "Quite a lot of functions in there. We can't see that any of them are particularly redundant. You need them all in some form."
He said the back office staff "are not just disposable assets you can chuck away" but key links in protecting the public and bringing criminals to justice. He acknowledged some non-frontline jobs did not have to be done by officers. But he insisted some areas should be off-limits. "I would want police officers involved in training detectives and people driving cars," he said.
Police minister Nick Herbert said: The Government will continue supporting forces by scrapping bureaucracy and driving more efficient procurement."