The head of the Association of Chief Police Officers has warned that chief constables may resign if a Conservative government tries to place forces under local political control.
Sir Hugh Orde said that police independence was vital to public trust and democracy.
The Tories want directly-elected commissioners to be able to hire and fire police chiefs and set force budgets in England and Wales.
Sir Hugh, who was formerly chief constable of the Police Service of Northern Ireland, was appointed president of the Acpo earlier this year.
In an interview with BBC Radio 4's Today programme, he said: "Operation independence is absolutely critical.
"Even the perception that the police service of this country... is under any political influence, I think that suggests you cannot argue that you are a proper democratic society. It's as simple and as stark as that.
"Every chief officer fully understands the need to be held accountable.
"We must be operationally independent in terms of how we deliver policing. We should not be influenced by anyone who has any potential or suggestion for a political basis."
He said that he "would not be surprised" to see chief officers resign and that if still a chief officer, he would do the same.
He called on politicians to focus on plans to replace the current 44 forces with larger regional ones better placed to deal with complex crimes including terrorism and internet-based fraud.
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