Two police forces are considering outsourcing neighbourhood patrols and some investigations to private firms, according to the Guardian.The paper said West Midlands and Surrey police have asked security firms to bid for contracts, worth £1.5bn over seven years, to run some services that are currently carried out by officers.
Successful firms would have a wide range of responsibilities, including detaining suspects and responding to incidents, but would not be able to arrest suspects.
In a briefing note sent to companies, which was seen by the newspaper, all services that "can be legally delegated to the private sector" are potentially up for contract.
Administrative roles, such as legal services and managing forensics, are also set to be outsourced in the sector shake-up.
The move will spark fears about privatisation within the police force.
Ben Priestley, Unison's national officer for police and justice, told the Guardian: "Bringing the private sector into policing is a dangerous experiment with local safety and taxpayers' money.
"We are urging police authorities not to fall into the trap of thinking the private sector is the answer to the coalition's cuts.
"The fact that the Home Office is refusing to publish its business case - even under FOI (Freedom of Information Act) - speaks for itself.
Critics say privatisation will mean that the police will be less accountable to the public, with people unable to go to the Independent Police Complaints Commission over problems.
"The only winners are private companies and shareholders who make profits at the expense of local services," Mr Priestley said.
A West Midlands police authority spokesman told the newspaper: "Combining with the business sector is aimed at totally transforming the way the force currently does business - improving the service provided to the public.
"The areas of service listed in this notice are deliberately broad to allow the force to explore the skills, expertise and solutions a partnership could bring."
A Home Office spokesman said: "Private companies will not be able to arrest suspects, and they will not be solely responsible for investigating crime. Many forces already use the private sector to run custody suites so that officers can be deployed elsewhere.
"We support the police in considering the value of private sector partnering to achieve cost savings and better services for the public."