Civilians working for police forces across the country are being used to investigate serious crimes including rape and murder in a cost-cutting initiative.

Police leaders and politicians have voiced their concern after discovering that some of the civilian investigators have come from jobs at supermarkets like Tesco and Marks & Spencer, or straight from college, having received as little as six months training.

The Daily Telegraph understands that hundreds of the civilian detectives are being used in half of all the 43 forces in England in Wales, carrying out inquiries into murders, rapes, burglaries, robberies, fraud, assaults and vehicle crime.

The initiative was started with small numbers of workers four years ago, and was designed to cut costs and free up frontline officers from desk-bound administrative roles.

However the civilian investigators are taking on an increasingly large number of serious crimes and rank-and-file police claim their traditional role is being undermined.

Inspector Steve Williams, of the Police Federation's National Detectives' Forum, said: "Some civilian investigators are dealing with low category murder cases and interviewing rape victims. You have people coming from college or Tesco, given minimum training and told to get on with it. That can't be right."?

Det Con Ian Spain, of Humberside Police, said that his force employed 50 civilian investigators.

"There are people who have come from working at M&S taking their first statement from someone who is a rape victim and then going off sick," he said, adding that the lack of expertise could lead to court cases being lost.

Det Sgt Ray Lewis of the Met that civilian detectives were the product of so-called "workforce modernisation".

"It is really about saving money - senior officers looking for ways to balance the books", he said.

Pilot schemes started on the use of civilian investigators in 2005 and chief constables have expanded the schemes, saying they have been successful in freeing up warrant card holding detectives and saving money.

But critics say that in some forces the project was being applied in an "ad hoc and haphazard" way.

Chris Huhne, the Lib Dem home affairs spokesman, said: "The public rightly expect that the investigation of the most serious crimes is undertaken by fully trained, experienced officers."?

In Greater Manchester, the first homicide squad made up entirely of civilian investigators has been launched, to work on the lowest-level murder cases where alleged offenders may be known to the police.

A total of 20 of the 23-strong squad are retired police officers. The three others, who have never held a warrant card, previously worked as a civil servant in the Department for Work and Pensions, for the RSPCA and in a solicitor's office.

Simon Reed, the vice-chairman of the Police Federation, has criticised Manchester police for forming the unit.

"They do it because it is much cheaper than investing in CID, which is what forces need to do. Could you imagine if they did this with nurses? Imagine patients were being looked after by retired nurses and people who have never been nurses. Both are skilled jobs, but there doesn't seem to be the same value on the skills detectives have."

However so far the unit has dealt with 33 "category C" murders and pressed charges in 28 of them, a detection rate of 85 per cent - higher than the overall Manchester Police rate of 77 per cent. It has also saved approximately ?500,000 on wages.

One of the investigators, Martin Rigby, a 49-year-old retired detective who served 18 years in CID, said: "It is disingenuous of the Police Federation to give the impression that we are a bunch of milkmen and window cleaners who don't know their **** from their elbow when it comes to solving murders. That's clearly not the case.

A Home Office spokesperson said: "The deployment of police officers and police staff to CID and other frontline units is an operational matter for the Chief Constable of each police force.

"There are many examples of police staff working in investigative roles, these include retired officers re-employed as staff, and staff who have been recruited with specialist investigative skills, for example in financial investigation, as well others employed to assist investigative work."