An ACPO spokesman said:
'Policing and Crime Commissioners (PCCs) represent probably the biggest change to policing since 1829. They introduce, for the first time, a directly elected person, in the majority of cases on a party ticket, to the British policing model.
'The police service continues to work with government to address legitimate concerns and to prepare the service and communities for the transition to PCCs in the autumn. A policing protocol helps to define roles and underline the independence of the chief constable to take professional decisions about how to deliver policing, held to account by the PCC.
'Tony Melville has made a personal decision today regarding his position as chief constable. He has served the public with distinction for 34 years and chief officers wish him well for the future.'