Volunteers will for the first time take on detective work after Essex Police advertised for special constables to help on major cases, including rapes and murders.
In an advertisement, the force said it was launching two new roles "where you can volunteer your time to solve serious crimes including murders, stranger rapes and fraud and corruption cases".
One voluntary sector figure said the move demonstrated that the government had failed to understand that volunteering was an investment in communities and not an excuse to get public services on the cheap.
The advertisement says that volunteers will be trained alongside special constables, before taking on further training with the force’s Major Crime Unit and Serious Economic Crime Unit.
Volunteers would be expected to work for 16 hours a month, the advertisement says.
Volunteers are widely used in the police service to provide support to police constables and have been deployed in specialist roles in some forces, including for the National Crime Agency in areas such as financial crime and cyber security.
But this is believed to be the first time a police force has used volunteers to support the investigation of major crimes such as murders or rapes.
This comes soon after the announcement of the NHS Long Term Plan last week, which called for an expansion of volunteering in the health service.
Rita Chadha, chair of Barking & Dagenham Council for Voluntary Service, said the latest use of volunteering in the police service showed the government had "failed to understand that volunteering is an investment in communities and individuals and not an excuse to get public services on the cheap".
She said: "This is a serious concern about how policing across county lines might be subject to volunteer interventions that may or may not cause conflict within communities, lead to a rise in community tensions, allow for services to become less reliable and, worst of all, mean that we move from a system of policing by consent to a model of policing if you want to."
Karl Wilding, director of public policy and volunteering at the National Council for Voluntary Organisations, said volunteering in areas where high levels of expertise were required or where it might be emotionally extremely challenging was not unusual.
"We have to be very careful that we don’t presume that volunteer is a synonym for amateur, because it is not," he said.
"If volunteers can help to improve the outcomes of public services, I think their involvement is a good thing.
"However, if your primary motivation for involving those volunteers is budget pressures and wanting to save money, I think that’s a bad idea."