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, it was reported.

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.

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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.

A West Midlands police authority spokesman said: "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."

He said not all the activities listed would necessarily be included in the final scope of the contract, but if the force added other activities later a "new and costly procurement exercise" would be needed.

The Home Office is believed to have given its backing to the proposal. A Home Office spokesman was unavailable for comment last night.

West Midlands is already planning to cut 2,764 police jobs over the next three years.

Ben Priestley, Unison's national officer for police and justice, which covers many police civilian staff, said it was alarmed by the programme. He said: "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 [the Freedom of Information Act] - speaks for itself.

"Privatisation means that the police will be less accountable to the public. And people will no longer be able to go to the Independent Police Complaints Commission if they have a problem. When a critical incident happens, a force's ability to respond will be severely compromised. The only winners are private companies and shareholders who make profits at the expense of local services."

Other forces have been exploring the services that might be offered to the private sector.

Cleveland police have a 10-year contract with IT firm Steria to provide call handling, front desk staffing, finance and training. Reliance security runs Cleveland's custody suites.

Avon and Somerset had a contract with IBM, called South West One, which suffered problems in its first three years. Some services are to be taken back in-house. Cheshire has a more traditional contract with Capgemini to provide finance, facilities and fleet management.