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Boris Johnson has suggested huge parts of the Metropolitan Police could be privatised to cut costs.

The Mayor said an “irreducible core” of duties would be protected but claimed private firms could run some Scotland Yard services “without making the thin blue line any thinner”.

The Met is struggling to fill a budget black hole of £250 million. Mr Johnson’s new deputy mayor for policing, Stephen Greenhalgh, is tasked with cutting costs.

Mr Johnson’s comments come after the West Midlands and Surrey forces set out radical plans for private firms to take responsibility for investigating crimes, patrolling neighbourhoods and even detaining suspects. But in a written answer, the Mayor said any private contracts would be more likely to involve back-office jobs.

He said: “I am committed to protecting the public service ethos of the MPS and support the Commissioner’s view that there is an irreducible core to policing that can never be outsourced.

“However, I do believe that competitive tendering of contracts beyond this provides a means of cutting costs considerably without making the thin blue line any thinner. It is therefore something that should be explored.”

Patrolling by community support officers, developing cases, responding to incidents, victim and witness support, managing high-risk individuals or intelligence and engagement with the public, as well as managing forensics and the vehicle fleet, finance and human resources could all be up for grabs.

However, the Mayor will be under pressure to maintain the public service ethos of the biggest police force. Labour’s policing spokeswoman Joanne McCartney said: “Policing is not for profit, it is about public service and keeping us all safe.

“We urgently need clarity as to what he considers ‘back room’. Does he mean 999 call centres and custody suites? These are core policing roles the public would not want privatised.”

Met Commissioner Bernard Hogan-Howe has said he believes investigating and patrolling should be the work of police officers.

“It’s a very serious issue to deprive someone of their liberty and place them before a court. For me that must be carried out by warranted officers.

"I think business can help improve our efficiency and effectiveness in some areas... such as managing our fleet and our back-office functions.”

Home Secretary Theresa May, who has imposed a 20 per cent cut in Whitehall grants to forces, has said front-line policing can be protected by using the private sector to transform other areas.