Crime rates could jump as one in 10 police officers is axed under Government spending cuts, an official inquiry has warned.
Rank and file officers said "our worst fears" had come true after being told more than 34,000 police jobs will be lost as part of the austerity drive.
About 16,200 police officers will be axed by 2015 amid warnings that crime could rise by 3%, Her Majesty's Inspectorate of Constabulary (HMIC) said.
Officer numbers will sink to their lowest level since 2001/02 as forces face "their biggest financial challenge in a generation".
Protecting frontline policing will be "very challenging" over the next 18 months, inspectors found.
Paul McKeever, chairman of the Police Federation of England and Wales, said police forces will struggle "to keep their heads above water as they try to deal with increasing demands and diminishing resources".
He added: "This will fundamentally change the way we police our communities, and an almost inevitable consequence will be a rise in crime rates as the population continues to increase and police numbers fall."
Up to 1,800 community support officers and 16,100 police staff will also go as part of an overall reduction of 14%, the study of 43 forces across England and Wales said.
"Forces will have to transform their efficiency if they are to protect frontline services," the report added.
Shadow home secretary Yvette Cooper said the potential loss of 2,500 frontline officers was an "irresponsible gamble", saying damage to policing from the Government's 20% cuts was "even worse than we previously feared".
Across England and Wales, police workforce levels will be reduced to 209,800 from 243,900 in March last year.
Research from academics suggests a 10% reduction in force officer levels could lead to a 3% increase in crime, HMIC said.
City of London police will be worst hit, with cuts of up to 19% in its gross revenue expenditure by 2015.
HMIC chief Sir Denis O'Connor added: "The police service must adapt to these changing times in order to deliver the best deal for taxpayers and they will need some support to do this."
Police chiefs said forces were facing "difficult choices" meeting national demands on policing.
Chief Constable Chris Sims, of the Association of Chief Police Officers, said: "Differing sizes, workforce mixes and financial starting positions mean each force is in a unique situation. But alongside huge effort to drive out cost we will see a smaller workforce and significant changes to the service we offer.
"Chiefs understand the policing sector cannot be immune from cuts taking place across the public sector. To be successful in preserving our service to the public we need the freedom to challenge the way we operate and, rather than crude numbers, focus on outcomes that keep people safe."
Labour leader Ed Miliband said he was "shocked" by the report, adding: "I think that really raises big issues for our society and big issues about how we ensure that we give people confidence."
Ms Cooper added: "The Home Secretary has said the cuts don't need to affect the number of officers or frontline services.
"But the independent inspectorate's report shows that is wrong. Many forces have no choice but to cut frontline officers because of the scale and pace of the Government cuts.
"Theresa May has put chief constables up and down the country in an impossible position."
David Cameron said yesterday that the phone-hacking crisis and allegations of police payments "calls for us to stand back and take another, broader look at the whole culture of policing in this country".
Anger has been building in frontline policing since former rail regulator Tom Winsor said the most wide-ranging analysis of forces pay in 30 years showed more than £1 billion of savings should be made.
Deputy Prime Minister Nick Clegg admitted the cuts would be "incredibly difficult".
He said: "It's not possible to somehow ringfence the financial pressures that exist on all public services.
"But there are a lot of things we can do to make sure the effect is not damaging on the front line."
The Government responded to criticism by saying it was cutting out "needless bureaucracy".
Crime and security minister James Brokenshire said police "can and are rising to the challenge" by reducing costs from the back office.
"HMIC predicts that by March 2012 the proportion of the police workforce working in frontline roles will be higher than it was in March 2010," he said.
"Every force should be driving through efficiencies, rooting out wasteful spending and cutting crime."