Staff employed by a private company are expected to be running reception areas and handling some evidence at police stations on Teesside within two years.
It is part of a £25m modernisation scheme under the Private Finance Initiative (PFI) which will see three new police stations built.
Up to 30 Cleveland Police employees will have their jobs transferred to a private company.
But unions have voiced concern at the long-term security of staff contracts.
Cleveland Police wants to build three police stations - at St Hilda's in Middlesbrough, one in South Bank and one at Kirkleatham near Redcar.
The plans are to also create 25 extra cells, a new front desk and custody suite at Hartlepool.
Joe McCarthy, chief executive of Cleveland Police Authority, said: "The PFI approach where we work with a private sector partner will attract government funding which would not otherwise be there - if we were not to go down this route it would cost us in excess of £20m a year."
In return for putting the money in to help build the new stations the private company will part own them and also partly run them.
But service sector union Unison said workers are concerned their contracts will be handed over to a new employer without security for their long-term future.
A Unison regional spokesman said: "They signed up for a job with the Cleveland Police Authority and they are very happy to work for the police - they are much less happy at the prospect of being employed by somebody else entirely which is not a public authority.
"We are concerned about future pay and conditions of service.
"We are asking that part of the contract with the private contractor should be that pay and conditions be preserved."