The Home Office came under pressure on Sunday to defend the effects of its policing cuts after it emerged that two police forces were proposing to hand over sensitive operational duties to a private contractor to help meet savings targets.

Keith Vaz, who chairs the home affairs select committee, has written to Theresa May, home secretary, to ask her how companies involved in police work will be held to account in the event of more cash-strapped forces turning to private contractors.

The issue was highlighted by a joint call for bids issued by West Midlands and Surrey police forces, which are planning a £1.5bn contract to deliver services such as having a role in criminal investigations, patrolling the streets and helping to detain crime suspects.

The contract note – which lists proposed changes only – goes well beyond the administrative functions carried out for various forces by companies such as Cap Gemini, IBM and Steria, and into traditionally no-go areas of operational policing.

Yvette Cooper, shadow home secretary, said she feared that in a drive to meet the government’s 20 per cent cuts to central police funding, chief constables had been pressed to “cross a line”, allowing private involvement in core public policing activity.

“Victims need to be confident that decisions on whether to investigate crimes or pursue particular criminals are made according to the public interest, not the private interest of a company,” said Ms Cooper. “And the public need to be confident that public streets are being patrolled in the interests of the community, not the company.”

The Police Federation, which represents rank-and-file officers in England and Wales, is even more concerned about the potential changes.

Simon Reed, the federation’s vice-chairman, believes the Home Office – which is pushing through the budget cuts – is fixed on the view that the public sector on its own cannot work, and that a public-private partnership is necessarily more efficient.

“Anything you make cheaper will be more efficient – the question is, will it be more effective?” said Mr Reed. “In the end, there are roles that the state must do, whatever the cost, and policing is one of them.”

Others argue that the country’s 43 forces have existed for too long with financial inefficiencies, and are in need of expert overhaul.

Mark Reckless, a Conservative MP and former member of Kent police authority, told the Financial Times: “What strikes me is how conservative with a small ‘c’ policing has been, and how little innovation there is.

“Outsourcing a criminal investigation or a custody duty is not distasteful.”

Mr Reckless, who is also a member of the home affairs committee, added that the introduction of directly-elected police commissioners – central to the coalition’s policing policy – would help to ensure that decisions on how and where to bring in the private sector would be made with full accountability.

“One of the best outcomes of moving towards elected crime commissioners is that they will be able to make a judgment about what the public will be prepared to accept and what is appropriate,” he said. “By comparison, police authorities are relatively weak and diffuse and find it harder to make these decisions.”

The security company G4S recently signed a £200m contract to staff and build a police station in Lincolnshire, a deal that was the forerunner for the West Midlands and Surrey plans.

John Shaw, the company’s managing director for policing services, said that even potentially controversial proposals in the West Midlands contract note – such as companies providing staff to protect crime scenes – would save forces money by freeing officers from basic jobs.

“Would you rather have a £35,000-a-year police officer doing this or would you prefer someone else whose job it is to do man-guarding?” he asked.

Will Tanner, a police and criminal justice researcher at Reform, a centre-right think-tank, argued that this sort of private sector involvement was a sign of long-overdue innovation in policing.

“It just shows that when you apply financial pressures to a service, it gives them impetus in quite a concentrated way and you bring about new thinking,” he said.

“We’ve got the best police force in the world, so we should be trying to follow that success and make it even better.”