Spending cuts threaten to undermine the financial sustainability and operational viability of some police forces, according to a report from the official policing watchdog.
HM Inspectorate of Constabulary (HMIC) said forces would need to make major changes to the way they worked in order to drive down costs in future years.
The result was likely to be a police service with fewer numbers and "perhaps less visible," it said.
It added: "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."
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.
West Midlands Police was graded as 'outstanding' in the report for its efficiency in matching resources to demand.
The force was one of just five across the country that was graded as 'outstanding', with another 29 rated as 'good', eight 'requiring improvement' and for the first time one force - Humberside - has been rated as 'inadequate'.
Her Majesty's Inspector Mike Cunningham, who led the inspection, said: "Police forces have been through change on an unprecedented scale since 2010.
"It is a tribute to the leadership of the police service and to officers, PCSOs and staff in all forces that the service has, on the whole, been able to absorb that change while measured crime has continued to fall and public satisfaction with the police has been maintained.
"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."
Just last week, it emerged the number of sexual offences in the West Midlands rocketed by 24 per cent last year while violent crime was also on the rise.
Overall offences in the region increased by two per cent, according to the National Crime Survey which is based on interviews with members of the public.
The survey showed a drop in robberies of seven per cent and domestic burglary saw an eight per cent decline.
Thefts were also down by 16 per cent along with drug offences, down by 14 per cent - although shoplifting increased by nine per cent.
Mr Cunningham added: "Forces have made great strides in assessing the current demand for their service, however they need to improve their ability to forecast demand.
"Typically, forces think in terms of numbers of officers and staff when developing workforce plans, rather than their skills and capabilities that will be required in the future.
"They need to start building their capability now, informed by a clearer understanding of future demand."