The five former top police officers in Britain have attacked the Conservatives’ record on crime, saying that “resources drained to dangerously low levels” have contributed to a “feeling of lawlessness” and saying confidence in the police has to be urgently restored.

The attack came from a string of commissioners of the Metropolitan police, Britain’s biggest force, who claimed the crisis was so deep that “the public have perilously low expectations of the police today”.

The letter is signed by Lord Condon, Lord Stevens, Lord Blair, Sir Paul Stephenson and Sir Bernard Hogan-Howe. It says: “The reduction of police and support staff by more than 30,000, the virtual destruction of neighbourhood policing and the inadvisable undermining of lawful police powers such as stop and search, have taken their toll. Common sense suggests that these factors have contributed to the feeling of lawlessness generated by knife murders and ‘county lines’ drugs.”

The letter, published in the Times, warns of an “emasculation of British policing” in recent years and says a royal commission is needed.

It was the latest warning about the crisis in policing, with the chief inspector of constabulary also warning on Thursday that policing is under huge strain, struggling to meet rising demand and that radical reform is needed as well as more officers.

In his annual state of policing report, Sir Tom Winsor said the criminal justice system was, in parts, failing, and said solutions lay in more rehabilitation, more crime prevention, and increased innovation and efficiency.

After years of funding cuts resulting in 20,000 fewer police officers, the chief inspector, who is deeply unpopular among many rank and file officers, who view him as the executor of the Tories’ police cuts, said they had influenced crime levels.

Winsor said homicides were rising and the fall in crime had stalled, with the poorest areas suffering the worst cuts. “There are indications that some forces are straining under significant pressure as they try to meet growing, complex and higher-risk demand with weakened resources.”

The wider criminal justice system was “malfunctioning”, “dysfunctional and defective”, he added.

Policing was under extra pressure to pick up services normally covered by other services that had been cut under austerity.

Winsor said the public “through their elected representatives” needed to decide how much money policing should receive after years of cuts, which were only recently being reversed.

Winsor called for greater rehabilitation of prisoners because many jails “were full of people who were ill rather than bad”.

Reform of policing was urgently needed, he said. “If they don’t, the windspeed of police reform will fall to a flutter, leaving the police service increasingly unable to meet the demands it faces. The inevitable legacy of such an approach 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.”

Winsor spoke after the frontrunner for the Conservative leadership, Boris Johnson, pledged to recruit an additional 20,000 police officers at an estimated cost of £1.1bn a year. He welcomed Johnson’s pledge but suggested the cost would rise every year, adding that investment was needed in technology and innovation to make policing more efficient.

“The police undoubtedly do need more people,” Winsor said. “Not all of that should be spent on hiring people. Some of that … should be spent on technology so they can be as efficient as possible.”

Asked about Johnson’s promise on officer numbers, Winsor said: “It may not be the most efficient and effective way of spending £1.1bn … every year.”

He added: “The police can be far more productive if they are given the kit that they need. What I think the government and public want is the productivity of 20,000 officers of the past, that doesn’t mean to say they need to have 20,000 people.”

He said changing crime and technology had changed the dangers children faced. “Most children are now more at risk in their own bedrooms then they are on the streets. This type of offending is not just about child sexual abuse and fraud, but radicalisation, harassment and stalking too,” Winsor said.

Winsor, who was a controversial choice to become chief inspector when given the job by Theresa May when she was home secretary said, since 2010, there had been more reform to policing than at any time since the 19th century.

Martin Hewitt, the chair of the National Police Chiefs Council, said: “The findings make clear that we can only deliver effective justice if all of the criminal justice system works together.

“To do that, all those with a part to play must be properly resourced, and as the report points out, that isn’t currently the case. Sir Tom makes a compelling case for long-term sustainable funding.”