Fresh spending cuts threaten to undermine the financial sustainability and operational viability of some police forces, the official policing watchdog has warned.
HM Inspectorate of Constabulary (HMIC) said forces will need to make major changes to the way they work in order to drive down costs in future years.
The result, it said, is likely to be a police service that looks different in five years' time - with fewer numbers and "perhaps less visible", while maintaining neighbourhood policing is likely to be a "challenge".
The findings echo recent warnings by senior officers - including the Metropolitan Police Commissioner Sir Bernard Hogan-Howe - about the impact of further cuts to police budgets expected in next month's spending review.
HMIC said that having been through change on an "unprecedented scale" since 2010, forces in England and Wales were preparing to lose another 7,400 officers over the next five years as well as 1,300 community support officers and 3,500 other staff.
However it found that the rigour of financial planning varied considerably from force to force, while most had a "weak understanding" of the future demand for police services.
In terms of efficiency, the number of forces judged to be in need of improvement has risen from three last year to eight, while for the first time one force - Humberside - has been rated "inadequate".
"The police service has faced a period of major change since 2010, particularly in reductions to its budgets and workforce," the report warned.
"There are no signs that that challenge will diminish in the years ahead with continuing reductions in public spending.
"As police forces face further reductions in their income, the shortcomings in the efficiency of some will draw those forces closer than necessary to the point at which their financial sustainability and operational viability may be in jeopardy."
It was "conceptually possible" that even efficient forces could struggle to remain viable, the report said.
While some forces had achieved spending cuts during the last parliament without radical reform of the way they operated, the report warned that approach would be increasingly difficult to sustain.
It said that police needed to keep pace with the growing use of technology by criminals. However, their computer systems were "generally weak and ageing" and lagged behind those used by the public.
The report also stressed the need for public reassurance as the profile of policing changed.
"Policing in five years' time is likely to look different to now and to 2010: smaller, less costly and perhaps less visible," it said.
"Maintaining visible neighbourhood policing, rather than becoming a mainly reactive service, is likely to be a challenge - and the public will need reassurance if policing becomes less visible."
Mike Cunningham, who led the inspection, said: "Police forces have been through change on an unprecedented scale since 2010.
"The next five years will be more challenging for forces as they strive to make further reductions in budgets and workforce, while dealing with increasingly complex crime. Policing is entering uncharted waters."