Current police numbers are not "sustainable" in the face of budget cuts, a senior officer will warn later.
President of the Association of Chief Police Officers Sir Hugh Orde will tell a conference it is "misleading in the extreme" to claim otherwise.
But he will urge ministers to make structural reforms, rather than "salami slicing" the police to save money.
The home secretary will tell forces they will no longer have to meet a set of minimum standards created by Labour.
Theresa May and Sir Hugh will address senior officers from England, Wales and Northern Ireland at Acpo's annual conference in Manchester on Tuesday.On the beat
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.
Speaking on the BBC Radio 4's Today programme, Sir Hugh said 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 out efficiencies would be a good starting point, he said.
Later, Sir Hugh will say 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.
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I am confident that colleagues will work tirelessly to achieve savings by collaborating
The Acpo president will also say 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 will say.
He will say forces might look to merge units and back-office functions, or share expensive equipment, but he will warn that police force mergers are unpopular with voters who fear losing their "local" police.
Speaking on the Today programme, policing minister Nick Herbert said "extensive cuts" to the police budget were inevitable but front-line posts should be protected.
He said central procurement, collaboration between forces and gettting 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 5 live.Force 'independence'
Speaking after Sir Hugh, the home secretary will announce 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 will also ask 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 will say.
"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.