A "compassionate" and "reasonable" debate on policing is needed in the wake of the 7 July bombings, Britain's top policeman is expected to say.
The public must decide what kind of police it wants, Sir Ian Blair says
Metropolitan Police chief Sir Ian Blair will use the annual Dimbleby lecture to argue the public needs to decide what kind of police it wants.
Without such guidance, police are being left to develop policies such as shoot-to-kill in private, he warned.
This could land them in political controversy, he told the Guardian.
Making his first public comments since the government's failed bid to extend the time terror suspects could be held without charge to 90 days, Sir Ian voiced "frustration" at the public "silence" on what it wanted the police to do.
He said police work was being hampered by the lack of a proper examination of what they were for - whether it was to fight crime or fight its causes, to build stronger communities or enforce zero tolerance.
If the public did not decide what kind of force it wanted, then the police would drift into deciding on their own, which would not be "right", he added.
And he criticised politicians for limiting debate to police numbers and calls for "bobbies on the beat".
Meanwhile, the force was "restless for change" and he wanted to "get on with it", he said.
Sir Ian said the police, traditionally a "silent and separate" organisation, was finding itself at the heart of political rows without any intervening public debate.
He cited the controversies over the Metropolitan Police's shoot-to-kill policy in the wake of the London attacks and the shooting by police of Brazilian Jean Charles de Menezes.
Police had also found themselves in the spotlight over their involvement in attempting to persuade MPs to back the government's bid to allow terror suspects to be held without charge for 90 days.
He said there should be more parliamentary debate about policing and suggested encouraging the further education sector to provide courses on policing.
And he said neighbourhood policing would be key to reform.
He added that Britain had too many police forces and warned a single unified police structure for London at least had to be in place before the Olympics in 2012 to enable it to police the event properly.