The Police Federation of England and Wales said constables "should never face the possibility of being removed from their job due to political reasons or interference".
Chairman Steve Williams said: "All police officers should have the operational independence afforded to them by the office of constable - the principle that they are accountable to the law and the law alone.
"Police officers must be free to carry out their duties for the benefit of the public and to be free from compromise they must be sure their actions could not be improperly held against them in the future."
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