The Metropolitan police are to stop investigating many lower level crimes as a result of spending cuts, a senior police officer has said.

On Monday, it was reported that the UK’s largest force would no longer look into many reports of crimes, including burglaries, thefts and assaults, where there was judged to be little prospect of identifying a suspect.

The plan has been denounced as a “green light to thieve” but the Metropolitan police said it needed to balance the books.

The deputy assistant commissioner, Mark Simmons, said: “With the pressure on our resources, it is not practical for our officers to spend a considerable amount of time looking into something where, for example, the value of damage or the item stolen is under £50, or the victim is not willing to support a prosecution.

“We need our officers to be focused on serious crime and cases where there is a realistic chance that we will be able to solve it. We also want them to be available to respond to emergencies and go to those members of the public that need our help the most.”

The force has said it must save about £400m by 2020 to make up for a real-terms budget cut, and says cuts to the number of officers it can deploy are likely to continue.

“We must prioritise our resources to be able to cope with the demand so our officers can be in the right place at the right time to help the public.

“The recently introduced crime assessment policy is helping us to do just that. By empowering our officers and giving them a consistent policy, they are making judgments about whether it would be proportionate to continue further with an investigation in some lower level crime,” Simmons said.

The policy’s existence was first reported by the Sun, which said it would lead to hundreds of thousands of crime reports not being investigated unless a victim’s report identifies a suspect.

The former Met Det Ch Insp Mick Neville told the Sun: “This is justice dreamed up by bean-counters in shiny suit land … Few professional criminals target people who know them, so the worst villains will evade justice. Not investigating high-volume crimes like shoplifting with a loss of under £50 will give junkies a green light to thieve.”

Chris Hobbs, a former special branch officer, said the move was appalling. “While it could be argued that this is an extension of current policy, where much crime is quickly screened out, I have to say I find this absolutely beyond the pale. It really does give the green light to criminals do as they please, especially if they are hooded or masked.”

Scotland Yard said in a lengthy statement on Monday that it would investigate crimes where there were leads to follow and that more serious crimes would always be investigated.

Simmons said: “Of course, we are not talking about things like homicide, kidnap, sexual offences, hate crime or domestic violence, but the lower level, higher volume offences such as shoplifting, car crime and criminal damage.

“This is not to say these cases will not be investigated further, however, by applying the assessment policy we will be able to determine very quickly if it is proportionate to do so.”

The Home Office has insisted its 2015 spending review protected the policing budget and said the Met had “had a broadly flat cash budget since then”.

Scotland Yard said increased costs meant it still had to save £400m, and that was on top of a £600m cut to its previous £3.7bn annual budget. The force said it was also having to deal with more crime and an increased terror threat.

The Home Office referred to a statement released in June that read: “Her Majesty’s Inspectorate of Constabulary has been clear that there remain significant efficiencies in policing to be delivered from digitisation, collaboration and improving workforce capability.

“The government is investing £175m in the police transformation fund this year to help all forces become more productive and manage pressures.”