Cuts to the police budget could compromise the safety of the public, warns the head of the Association of Chief Police Officers, Sir Hugh Orde.
Calling on the Government for greater clarity over its plans for the force, Sir Hugh said: "We understand the Government's determination to deliver a substantial programme of reforms across the public sector, but we cannot afford to get policing wrong, and unless greater clarity emerges in the very near future I fear that we run the risk of compromising the safety of citizens for reasons of expediency."
The Government's radical reforms for police forces will see budgets cut by 20 per cent over four years.
But Sir Hugh warned the Home Secretary, Theresa May, that the raft of changes on the horizon puts the force's impartiality at risk.
"The service of last resort is going through a period of substantial change," he said.
"Changes to accountability, changes to central structures and changes to pay and conditions, which if mismanaged could threaten the impartial model of policing that has existed for 180 years and is revered across the world."
Some of you are still setting your forces targets that we've scrapped nationally. Home Secretary Theresa May
Yet Mrs May hit back, insisting that the Government "really means business". She accussed police chiefs of sticking with targets she has scrapped specifically in order to free police from bureaucracy and get them back on the streets.
"Here's the problem - not all of you are following my lead," she told police chiefs today.
"Some of you are still setting your forces targets that we've scrapped nationally. And it's not only one or two chiefs who are reinstating targets and forms we have eliminated - officers up and down the country are telling me they're still having to record information at local level that we've stopped asking for at national level."
Opening the Association of Chief Police Officers (Acpo) conference, Sir Hugh said the public sector is facing the most challenging times in living memory.
"In short, we have a change programme that at one end will produce some of the most radical changes to police governance since 1829, and at the other will without question reduce police and staff numbers and pay," he said.
Fuelling further fears over police cuts, former rail regulator Tom Winsor said recently the most-wide ranging analysis of police pay in 30 years showed more than £1bn of savings should be made.
Sir Hugh said: "That is a huge challenge for Acpo, as the leaders of the service."
Shadow Home Secretary Yvette Cooper said Mrs May should listen to Sir Hugh's warnings.
She said: "This is a serious warning for the Home Secretary from one of Britain's top police officers, which she would be deeply unwise not to heed.
This is a serious warning for the Home Secretary from one of Britain's top police officers. Shadow Home Secretary Yvette Cooper
"David Cameron and Theresa May are taking big risks on law and order. Hugh Orde is right to point out that the Home Secretary is reducing police numbers and police powers but increasing the risk of politicisation.
"This endangers the centuries-old tradition of impartiality as well as the effectiveness of the police and it is communities that will pay the price.
"They are whipping up a perfect storm, cutting 12,000 officers, weakening police powers, risking impartial British policing with American-style elected commissioners, sowing confusion over national policing plans, and undermining police morale by bad handling of pay and pension reforms."