David Cameron has defended controversial plans to pay private firms £1.5 billion to carry out police work – after he was warned that foreign suspects are being allowed to walk free because of a shortage of interpreters.

The Prime Minister backed plans drawn up by West Midlands Police and Surrey Police to take hire private firms to carry out key aspects of policing.

Speaking in the House of Commons, he told MPs: “I don’t think there’s anything wrong with the police getting back office functions carried out by private sector organisations.”

In fact, the forces have sent briefing note sent to potential bidders lists a range services they could be expected to provide, which go far beyond “back office” tasks and include investigating crimes, detaining suspects, managing major incidents, patrolling neighbourhoods and disrupting criminal networks.

The Prime Minister was responding to a question from Birmingham MP Gisela Stuart (Lab Edgbaston), who highlighted the scandal of West Midlands Police letting suspects walk free.

It was revealed earlier this month that officers have been forced to release some arrested foreign suspects on bail because they cannot get interpreters for police station interviews.

The shambles was blamed on a new interpreter service that was supposed to save West Midlands Police £750,000 every year.

Ms Stuart asked the Prime Minister: “Is that the kind of service we can expect when our police forces are outsourced to private security companies?” West Midlands Police and Surrey Police plan to meet businesses interested in taking over responsibility for key aspects of policing at a conference for bidders on March 14.

Labour’s Shadow Home Secretary, Yvetter Cooper, said: “Public private partnerships can be very effective and of course police forces should pursue efficiencies. But they mustn’t cross the line which would put public trust or the principles of impartial British policing at risk.

“We have not yet seen the detailed proposals for the West Midlands and Surrey police, and we will scrutinise them in detail.” West Midlands Police has defended controversial plans to contract out some services to the private sector after insisting that the force needs to find ways to be “cheaper and better.”

The force is one of two asking for security firms to bid for contracts, worth £1.5 billion over ten years, to cover services which are currently carried out by officers. Successful firms would have a wide range of responsibilities, including ‘detaining’ suspects and responding to incidents, but would not be able to arrest suspects. They would also collect CCTV as well as guard crime scenes.

Assistant Chief Constable Marcus Beale said it was “common sense” to find new ways to be cheaper and better as the force strives to find £126 million of savings. He added that time consuming and low complexity work could now be done by private firms instead.