The Association of Chief Police Officers said "radical and fundamental" change was needed to cope with financial cuts faced by Forces.
West Midlands Police and Surrey have invited bids from private security companies to take over delivery of a wide range of services currently carried out by police officers.
The contract is worth £1.5 billion over seven years but could rise to £3.5 billion should other forces follow suite and put their services out to tender.
Greater Manchester chief constable Peter Fahy, Acpo's spokesman on workforce development, said private security staff were already patrolling public spaces and managing major publlic events.
He said: "Private staff monitor CCTV covering public space, private companies transport prisoners to and from court and store detectives detain shoplifters."
Writing in the Guardian, the former Metropolitan police commissioner, Sir Ian Blair, said "swaths of police tasks'' do not need to be carried out by fully trained officers, such as guarding prisoners, preparing routine witness statements and providing intelligence analysis to murder inquiries.
He said: "Many forces have employed their own non-police staff to undertake this sort of task but have been unable to do so in sufficient numbers because of the need to employ a fixed and ever increasing number of officers within a fixed budget.
"The tender offered by West Midlands and Surrey police signals a shift which would allow the private sector to provide staff who can carry out routine and repetitive tasks at cheaper rates and, perhaps most intriguingly, to provide temporary access to skilled staff - such as murder inquiry teams - which can be hired for incidents that are rare in most forces but for which all forces must permanently retain a group of very expensive staff.
"This would then allow the chief constable, satisfied that he or she has commissioned these kind of services at a cheaper rate, to spend more of the budget on those parts of the service that require, because of their complexity, their impact on public safety or their centrality to the police mission, to be carried out by fully warranted officers."
Unde the West Midlands and Surrey police scheme, a briefing note has been sent to firms offering them the chance to bid to run all services that "can be legally delegated to the private sector". They do not include those that involve the power of arrest and the other duties of a sworn constable.
The services include investigating crimes, detaining suspects, developing cases, responding to and investigating incidents, supporting victims and witnesses, managing high-risk individuals, patrolling neighbourhoods, managing intelligence.
The contract also offers bidders that chance to take over more traditional back office roles such as managing forensics, providing legal services, managing the vehicle fleet, finance and human resources, The Guardian reported last night.
Privatisation of the police service has long been a contentious issue. Last month it was revealed that G4S has won a £200m contract to build and run a police station on behalf of Lincolnshire police.
And last year the Daily Telegraph reported that hundreds of police officers were being funded by schools, shopping centres and other private companies to carry out specific policing tasks.
The moves come following a pledge by Theresa May, the Home Secretary, to slash policing budgets nationwide by about 20 per cent.
Many forces are struggling to meet the cuts and some senior officers believe it cannot be done without harming the frontline.