Up to 40,000 frontline police jobs could be axed if Government funding cuts go ahead, the Police Federation said today.
Simon Reed, vice-chairman of the body which represents officers in England and Wales, said cuts of 25% would "devastate" the police service.
It was "inevitable" that crime would go up, he added.
The West Midlands force could lose up to 1,000 officers as it makes cuts of £140 million, figures released by the Police Federation showed today.
Greater Manchester Constabulary has lost 221 officers since last December, and North Wales will lose 251 officers in the next four years, the figures showed.
Several federations - including those in the West Midlands, Cleveland, Gloucestershire, Warwickshire, Hampshire, Devon & Cornwall, Northamptonshire and Durham - all warned that "major incidents will get a lot worse".
Others predicted increases in anti-social behaviour and burglary.
Paul McKeever, chairman of the Police Federation, agreed that it would be "Christmas for criminals" as tens of thousands of jobs were lost and specialist departments, including those involved with child protection and domestic violence, "disappeared".
"We expect the Home Secretary to wake up to the reality that there are going to be real consequences in the future if these cuts are made," he said.
"The reality has come. The cuts are real. They are substantial and it can't but have a detrimental effect on the service we provide."
One estimate found 60,000 frontline and civilian jobs in the police service would be at risk if the cuts went ahead at 25%.
Mr McKeever said he was "surprised" that the police were not in the lower bracket of cuts, but instead had been told to prepare for reductions of 25% or more.
Based on figures provided by the West Midlands force, he said the Metropolitan Police could lose up to 4,000 officers.
Mr McKeever blamed "bad advice" from the Home Office and think-tanks which suggested that big enough savings could be made through efficiencies to justify a 25% cut.
And he warned Police and Criminal Justice Minister Nick Herbert that it was "time to put aside theory and deal with reality".
The funding cuts, and the desire of the Justice Secretary Kenneth Clarke to see more offenders dealt with in the community rather than with short-term prison sentences, would be a "volatile mix", the chairman said.
The "most vulnerable in society will be hit the hardest", he said.
Mr McKeever said he hoped the Government would listen "to the gravity of what they will be facing" and said there were no plans for any protests or marches by officers.
"We hope they will listen and see sense," he said.
Asked for his message for Home Secretary Theresa May, Mr McKeever said: "Fight the case at the Home Office; tell them there are consequences; wake up to the reality of what police officers are telling you.
"And, for heaven's sake, don't accept 25% for policing, because the public don't expect that.
"The public, when we speak to them, think we are naturally (in) the lower level of cuts, that the NHS, education and defence will be in.
"I think they've made an error there."
Earlier, Mr Reed told the Press Association: "The cuts, if they're going to be 25% - which we're led to believe they are - will devastate the police service.
"We've done a survey of our branch boards around England and Wales and what they're telling us is quite bleak.
"Forces will lose potentially hundreds of officers and nationwide that could be anything up to 40,000 officers within the next two, three or four years."
Asked if crime would rise if the cuts went ahead, he said: "We think it's inevitable."
He added that the 40,000 figure related to officers only.
Earlier, Mr Herbert said: "I understand the Police Federation wants to make its case and protect every job, but we must be careful not to frighten the public.
"Police forces can make savings. They can become more efficient. They can share services and procure equipment better."
The Home Office said future funding for the police would be decided by the spending review, which is due to report in October.
A spokesman said the Government's priority was to cut the deficit and get the economy moving again, and added that the police service would need to play its part in achieving that.
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