Police officers may have to take on back office roles in the wake of fresh budget cuts, crime commissioners have warned.
Forces are already said to be warning that they will have "no option" but to shift uniformed officers into support posts if a new round of austerity means civilian staff numbers are scaled back.
The Association of Police and Crime Commissioners (APCC) has submitted an analysis of the possible impact on the police service of possible cuts over the next five years.
The Home Office, which is responsible for handing out money which makes up the majority of police budgets, has been told to prepare proposals that would achieve savings of 25% and 40% by 2019/20.
The department said no decisions have been taken on police funding beyond the current financial year.
The APCC said its current modelling of possible changes suggests that police officer numbers could fall below 100,000 by 2019/20. Figures show that in March there were 126,818 officers working in the 43 forces in England and Wales.
The analysis said: "Even if uniformed staff reductions do not exceed the levels of natural turnover, the forecasts imply that police officer numbers will fall below 100,000 by 2019/20."
There are already signs of "unavoidable actions which may compromise some of the beneficial changes from the past", according to the report.
It said: "The best example is use of uniformed staff to undertake desk-based roles. Civilianisation has been one of the success stories of the last 15 years.
"There are no restrictions on the use of both voluntary and compulsory severance for non uniformed staff, but the options available on uniformed staff are more restricted.
"If overall establishments need to be further reduced - and there is no realistic alternative - some forces are already warning that they will have no option but to transfer police officers back into support roles."
Paddy Tipping, PCC for Nottinghamshire, said PCCs have made significant savings since they were elected in 2012.
"We know that more will be needed," he said, adding that other sectors and industries must share the responsibility of responding to the changing nature of crime.
Mr Tipping said: "With more savings needed there will need to be a fundamental rethink on how we are organised, the service we offer the public, and the roles and skills needed."
Controversy over the possible impact of new cuts erupted earlier this year when a senior officer suggested burglary victims may not always be visited at home.
The Home Office said future funding levels will be subject to the outcome of a spending review which will report in November.
Policing minister Mike Penning said: "Police reform is working. Over the last five years, frontline services have been protected, public confidence in the police has gone up and crime has fallen by more than a quarter, according to the independent Crime Survey for England and Wales.
"There is no question that the police still have the resources to do their important work. What matters is how officers and staff are deployed, not how many of them there are in total.
"The changes the Government have made since 2010 have made it easier for the police to do their job by cutting red tape, scrapping unnecessary targets, and giving officers the discretion to use their professional judgment.
"Decisions on the operational deployment of resources are matters for Chief Constables in association with Police and Crime Commissioners."