John Apter, who represents rank-and-file officers, hit out at the prime minister’s policies as home secretary after attending a knife crime summit at Downing Street.
He said the government had left public services “stripped to the bone” and police “firefighting” stabbings and murders.
“Whilst I fully accept we have to look forward, I think the government and the prime minister in particular has to shoulder some of the responsibility for the government’s failed policies,” Mr Apter added.
“What she’s done to policing is unforgivable and she’s now seeing the impact of that.”
Ms May provoked fury last month when she said there “was no direct correlation between certain crimes and police numbers” – which have fallen by more than 20,000 since 2010.
Following a spate of fatal stabbings across the country, she headed a meeting of police leaders, youth workers, academics and victims’ representatives on Monday.
Opening the Serious Youth Violence Summit, which will be followed by days of meetings between ministers and crime experts, Ms May said knife crime was a “top priority for government”.
“In recent months we’ve seen an appalling number of young lives that have been cut short or devastated by serious violent crime, including a number of horrifying incidents which took place just over this weekend,” she added.
“We cannot simply arrest ourselves out of this problem – this is a wider problem. It’s more deep seated and we need to have a more coordinated effort in response to it.”
The prime minister was also to meet privately with the families of knife crime victims.
The government has opened a consultation on a statutory “public health duty” for teachers, nurses and other public sector workers to raise concerns about children at risk from knife crime.
The prime minister called serious youth violence a “cancer” and announced a new ministerial taskforce and Cabinet Office team to coordinate work across government departments.
On Sunday, Sajid Javid enhanced stop and search powers that allow police to challenge people without reasonable grounds for suspicion in areas where violence is expected.
The home secretary hailed a £970m annual funding rise for the coming year but police leaders said the boost – mostly generated by council tax increases – is still not enough to cope with the demand they face.
Mr Apter said the government must support both policing and the public sector to fight the root causes of violence, adding: “The only way that something positive will come out of it is if there’s a genuine investment – not just a few million pounds, this is going to cost many, many millions of pounds, to even think about putting it right.”
Martin Hewitt, who became the new chair of the National Police Chiefs’ Council on Monday and attended the summit, said the funding increase would help forces carry out targeted patrols to disrupt gangs.
But he warned that the £970m funding increase was “positive but not enough to meet all the challenges that the service faces”, and said he would be pushing for more investment in the upcoming government spending review.
Association of Police and Crime Commissioners chair Mark Burns-Williamson said Monday’s summit was a positive step towards taking a “whole system approach” against violence, including youth services and public health.
But he added: “We will continue to build a compelling case for increases in central police funding to increase resources and allow for further investment in crime prevention, in the forthcoming comprehensive spending review. This will be vital if we are to reduce crime over the longer-term.”
Diane Abbott, Labour’s shadow home secretary, said there was no evidence that “random stop and search” would reduce violence.
“Passing responsibility to our hard-pressed teachers and nurses won’t work either,” she added.
“A genuine public health approach does work, as has been proven in Scotland and elsewhere. But the Tories’ entire programme of public spending cuts has contributed to the causes of rising violent crime.”
Yvette Cooper, chair of the Home Affairs Committee, said the government was failing to stop teenagers being killed on Britain’s streets.
The Labour MP told the House of Commons: “There has been a reduction of 50,000 people working on community safety and child protection.
“Children’s lives are being lost, it’s crucial that investment into prevention takes place.”
Responding during Home Office questions, Mr Javid said the 2019-20 funding settlement was the “biggest [ever] increase in police resources” and would lead to the “recruitment of 3,000 officers”.
The debate follows a spike in stabbings, including a series of knife attacks in north London, the death of a 40-year-old motorist at the wheel of his vehicle in Clapham and a fatal stabbing in Liverpool.
There were 285 homicides where the method of killing was by a knife or sharp instrument in the year to March 2018 – the highest number since records started in 1946.
The government’s Serious Violence Strategy cited drug dealing and social media as key drivers of rising stabbings, but critics have hit out at cuts to policing, youth and social services.