Serving police officers are being taken out of frontline roles and moved to cover the "back-office" functions of civilian staff who have been made redundant, according to leaked memos which show the perverse side-effects of budget cuts.

The decision by Warwickshire police authority – one of the smaller forces in England and Wales with 1,800 officers and staff – to draft up to 150 frontline officers into civilian desk jobs is expected to be followed by other forces grappling with a 20% cut in their Whitehall funding.

Police officers are Crown-appointed warrant holders and cannot be made redundant. They can only be "compulsorily retired" through an obscure regulation after more than 30 years' service, but civilian support staff do not enjoy such job security.

The leak comes as a second survey of police authority intentions carried out by Labour confirms that the police are heading for 27,500 job losses, including 12,500 police officers, over the next four years. Ministers have vowed to protect frontline policing from the impact of the cuts and a report by Her Majesty's Inspector of Constabulary to be published on Wednesday is expected to clear up the confusion over where the "frontline" can be drawn in the battle against crime.

The shadow home secretary, Yvette Cooper, said the Warwickshire situation showed that chief constables had been put in an impossible position: "It is now clear that when there is not the staff to help plan, co-ordinate or forensically investigate the fight against crime, then police officers will have to be taken off the streets to do this work.

"The government needs to take responsibility and recognise that the loss of 12,500 police officers and 15,000 police staff across the country is taking risks with public safety and the progress on crime and antisocial behaviour that was made over the last decade."

The decision by Warwickshire to redeploy frontline officers to roles such as staffing inquiry offices and control rooms and conducting routine visits to crime scenes was disclosed in a leaked memo by Richard Elkin, the force's human resources director.

He has written to all 860 back-office staff inviting those with more than two years' service to apply for voluntary redundancy: "Whilst the force manages the required reductions in the number of police officers, it has been agreed that some will be temporarily posted into police staff posts which are currently vacant, or which will become vacant following voluntary redundancy," says the memo.

The Warwickshire force faces losing 450 jobs out of its 1,800 strength to find savings of £23m in its £100m budget by 2015. The home secretary, Theresa May, and the police minister, Nick Herbert, have repeatedly said it is possible for savings to be found through cutting bureaucracy and back-office functions without hitting the frontline.

Ian Francis, chairman of Warwickshire police authority, has said that there are too many police officers in the county force for the new model of policing which is being implemented. "We don't like it, they [Warwickshire police federation] don't like it, I don't think the public like it, but at the end of the day we have no option," Francis has said.

Francis has predicted that other forces are also likely to draft frontline officers into support roles: "The simple matter is yes, we are going to lose policemen from the front line."

Simon Reed, vice-chairman of the Police Federation, said Warwickshire's example would be followed by other forces: "What is happening in Warwickshire will happen elsewhere simply because of the sheer amount of money being cut from budgets.

"When we lose staff in inquiry offices, control rooms or going to scenes of crime then this will happen."

Reed said the cuts would reverse a 10- year process of getting uniformed officers back into mainstream police roles: "It is a question of teamwork. We all depend on each other. The frontline depends on the back-office function."

But a Home Office spokeswoman insisted savings could be achieved without cutting the frontline. "We believe that police forces can make the necessary savings while protecting frontline services and prioritising the visibility and availability of policing," she said.

"Forces must focus on driving out wasteful spending, and increasing efficiency in the back-office. The effectiveness of a police force does not depend primarily on the number of staff it has, but rather on the way they are deployed."