Boris Johnson’s pledge to recruit 20,000 more police officers would not solve the mounting problems with crime and violence in the UK, the chief inspector of constabulary has said.

Sir Thomas Winsor warned that keeping the public safe was “not just about police numbers”, and said the Tory leadership favourite’s plans were “not the most efficient and effective way of spending £1.1bn on policing”.

“You can’t recruit them all that quickly, they take 18 months to three years to train and not everyone who wants to be an officer can,” Sir Thomas told journalists at a briefing on Thursday. “You’ve got to invest to be more efficient in the necessary technology and ways of doing things, and that will cost money too.

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“So it may be that while the financial commitment from the Treasury may be £1.1bn to begin with, not all of that should be spent on hiring people, some should be spent on technology so that they can be as efficient as possible. You can get the productivity, efficiency and performance of 20,000 police officers with fewer than 20,000 people.”

The chief inspector also questioned the capacity for training and assessing that number of prospective officers.

Chief Constable Mike Cunningham, CEO of the College of Policing, said an investment in police numbers alone “will not be enough to meet the future demands we face as a service”. “Investment is needed in how officers and staff are developed throughout their careers to create a consistent national standard of policing across the country,” he added.

Diane Abbott, Labour’s shadow home secretary, said: “It’s difficult now to believe Boris Johnson’s campaign promise to restore the cuts to police numbers when he has repeatedly voted for them as an MP.”

Police officer numbers have fallen by around 20,000 nationally since 2010 in the wake of swingeing cuts by central government, which are starting to reverse since Sajid Javid became home secretary.

The Police Federation, which represents rank-and-file officers, is among the bodies drawing a link between the reduction and rises in violence and other serious crimes.

Theresa May, who implemented the cuts as home secretary, has denied the link and Mr Johnson had not publicly challenged her position or voted against government budgets.

Amid debate over how Mr Johnson’s proposals would be financed, Sir Thomas warned that the £1.1bn cost would rise annually because of inflation and pay rises.

His annual assessment of policing in England and Wales found there had been a real-terms reduction in police funding of 19 per cent since 2010-11, leaving one police officer for every 480 people.

Cressida Dick contradicts Theresa May over police cuts

The chief inspector noticed that police spending in England and Wales represented only around 2 per cent of public expenditure, despite it being “among the most essential public services of all”.

He suggested that a “national debate” should take place about whether to increase police funding through taxation or other means. “It is apparent to everyone that police cannot meet all demand, so something has to give,” Sir Thomas added.

“Almost every day we hear about young people dying on the streets and the fear of violence. The public need to figure out if they want to pay more for our police to reduce that kind of offending.”

The report found that because of a flawed formula used to decide how much government funding is given to different forces, poorer areas have experienced the greatest cuts.

“Police in poorer areas are more stretched, people in those areas are therefore less safe,” the report said.

Sir Thomas said that cuts implemented by the Coalition and Conservative governments had also damaged public services vital for crime prevention work, including mental health services.

“When other public services fail that shunts more demand onto the police and that is wrong,” he added, warning that “picking up the pieces” of non-crime incidents was preventing officers from carrying out vital policing work.

Johnson visits the Thames Valley Police Training Centre in Reading (PA) (PA Wire/PA Images)

He said budget cuts had initially triggered increased efficiency but “there comes a point where they would need to be reversed because of changes in the cycle and pattern of offending”.

Sir Thomas cited rocketing online child sex abuse, stalking and fraud as among the increasing offences that must be tackled with improved technology, as well as more police numbers.

His report warned that police were being “overwhelmed” by the quantity of data on mobile phones seized for investigations, which could be processed with the aid of artificial intelligence.

The report called for “profound and far-reaching” reform of the structure of policing in England and Wales, which is currently split between 43 local forces and various regional bodies.

“There is now a pressing need for a single-system operation,” Sir Thomas said. “That does not mean creating a single national police force but it does mean that those forces should operate as a single system in all respects that require it.”

Without reform, he warned that officers would not be able to meet the changing demands they face, and there would be “unacceptable compromises in both the quality of service the police can offer the public and the level of public safety and security the police can uphold”.

Sir Thomas suggested that mandatory standards for the quality of policing could be introduced, alongside harsher punishments for technology firms that enable crime and abuse. “Fining large multinational corporations won’t do it, the directors of these companies who knowingly facilitate these things should face the loss of their fortunes and their liberty,” he added. “They’ve had too long to self-reform.”

Nick Hurd, the policing minister, said: “We have already made progress reforming the police system but recognise there is still more to do and are working with policing leaders across the country to build a smarter, more efficient system with crime prevention at its heart.