The Government's radical police reforms could threaten the nation's globally "revered" reputation for public safety, force chiefs are set to warn the Home Secretary.
Sir Hugh Orde will tell Theresa May a raft of changes to police structures, strategy and pay puts a 180-year-old model of impartiality at stake.
"The service of last resort is going through a period of substantial change," he will say. "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."
Sir Hugh opens the Association of Chief Police Officers (Acpo) conference by calling for the Government to offer greater clarity over its plans.
He will add: "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."
Mrs May is expected to defend the Government's cost-cutting stance before facing a question and answer session during the opening of the Harrogate conference.
Anger has been building since former rail regulator Tom Winsor said the most wide-ranging analysis of police pay in 30 years showed more than £1 billion of savings should be made.
Sir Hugh will tell the conference how "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 will say. "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, adding: "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. 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.
"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."