Current police numbers are not "sustainable" in the face of budget cuts, a senior officer has warned.
President of the Association of Chief Police Officers Sir Hugh Orde told a conference it was "misleading in the extreme" to claim otherwise.
He urged ministers to make structural reforms and not "salami slice" forces.
Home Secretary Theresa May told chiefs that cuts would be "big and tough to achieve" but minimum standards and targets would be scrapped immediately.
She said the Policing Pledge, introduced by Labour, created added bureaucracy and that she would be "ruthless in cutting out waste, streamlining structures and improving efficiency".
"We're not talking about a spending freeze or a reduction of 1% or 2%," she added.
Theresa May and Sir Hugh addressed senior officers from England, Wales and Northern Ireland at Acpo's annual conference in Manchester on Tuesday.
The BBC's home affairs correspondent Danny Shaw said ministers had so far skirted around the question of whether budget cuts would hit police numbers.
But he said Sir Hugh was now addressing the issue and seemed to be saying that they would.
Sir Hugh told the BBC that the "rub" came with the numbers.
"With 83% of the police budget being people, sadly we will lose people in my prediction over the next few years," he said.
But Sir Hugh warned against "alarmist" predictions.
"Some services will have to be reduced - I think I am very clear on that. Our role is to make sure they are the less critical ones, the nice-to-do things rather than the essential-to-do things," he said.
Reducing bureaucracy, more collaboration and driving up efficiencies would be a good starting point, he said.
Sir Hugh said a balance must be struck between the "understandable demand" for more officers on the streets and the "less visible, but equally critical" duties they perform.
The Acpo president said that chief constables face "hard choices" over where to make savings and the government must take the lead.
"I am confident that colleagues will work tirelessly to achieve savings by collaborating, but remain of the view that there is more potential if this is centrally led in a more strategic way," he said.
Forces might look to merge units and back-office functions, or share expensive equipment, but he warned police force mergers were unpopular with voters who fear losing their "local" police.
Policing minister Nick Herbert told the BBC "extensive cuts" to the police budget were inevitable but front-line posts should be protected.
He said central procurement, collaboration between forces and getting civilians to take over paperwork duties would help drive down costs.
"In the back office, it may well make sense, as some forces have done, to give those tasks to civilians and in that way, get sworn officers out on the streets," he said.
Dale Bassett, of think tank Reform, agreed civilians might be "better suited" and "cheaper" for some roles.
"It's really important that chief constables start to get out of this mindset that uniformed officers are the only way to go," he told BBC Radio Five Live.
Speaking after Sir Hugh, the home secretary announced plans to cut police bureaucracy by scrapping Labour's 10-point policing pledge, which includes commitments to respond quickly to calls and keep victims informed about their case.
Sir Hugh asked the government to give more detail on how plans to introduce locally elected police commissioners will work.
"There are many mixed views on whether some form of locally elected element adds value or not," he said.
"My position remains the same - the test is reconciling it with operational independence for policing.
"Squaring that circle is a matter for government, but in this world of hard choices, we have an absolute right to clarity on how this system will work."
Last week, Metropolitan Police Commissioner Sir Paul Stephenson said his force would "shrink" as a result of budget cuts.