Sir Ian Blair tonight described the shooting dead of an innocent Brazilian as a watershed in modern policing which demands a wide-ranging public debate to determine the future direction of the force.

The Metropolitan Police Commissioner was to use the annual Dimbleby lecture on BBC1, the first given by an officer since Sir Robert Mark addressed the nation in 1973, to ask what type of police force Britain wants in the wake of the July 7 London bombings.

According to a copy of his speech, he was to say: "The dreadful death of Mr [Jean Charles] de Menezes is a watershed. Until now, the police have discussed the strategy and tactics for using lethal force behind closed doors. That has to change. An open debate is now required."

Sir Ian's comments come as his force faces accusations that the bullets used to shoot Mr Menezes, 27, as a suspected suicide bomber on July 22, were of a kind banned in warfare more than 100 years ago because of the massive damage they cause.

Alex Periera, a cousin of Mr de Menezes, said he was "shocked and angry" by the latest allegations, which surfaced in The Daily Telegraph, that officers are armed with hollow-tipped dum-dum bullets which expand on impact.

The controversy strikes at the heart of Sir Ian's speech: he argued that in the absence of an informed public debate, police have been left to make decisions on such issues as shoot-to-kill and the detention of terror suspects on their own.

"Doctors are surrounded by people with views on the medical ethics of the sustaining of life at its outset and at its end; where is the similar informed, compassionate, reasonable debate about the sudden and absolute end of life at the hands of the police?

"The silence can no longer continue. The citizens of Britain now have to articulate what kind of police service they want."

Sir Ian described the traditional bobby, immortalised by Dixon of Dock Green and symbolic of a golden age of social unity, as a fiction. He argued that a balance must be found between the cherished ideal of a force of friendly, local officers and one capable of defeating terrorists intent on causing mass civilian casualties.

He believes the police are being left to pick up the pieces from the collapse of traditional society evidenced by the decline in influence of churches, voluntary clubs, trade unions and even park keepers.

He will say: "This has left many people looking – in the absence of anyone else – to the police service for answers to the degradation of communal life – for answers to the neighbours from hell, the smashed bus stop, the lift shaft littered with needles and condoms, the open drugs market, the angry, the aggressive and the obviously disturbed.

"We now have to make some choices. Society is demanding answers and actions to deal with feral children, hoodies and yobs; to the curse of drugs, to date rape and gun crime; to the smuggling of women for sexual slavery; to street robbery; to truancy, graffiti and drunken aggressiveness; to paedophilia, identity theft, organised crime and murder.

"At the same time, those choices must reflect what kind of police service is needed after July. Terror has changed its methods – or, more accurately, brought some existing methods to Britain for the first time.

"Britain remains a target of the highest possible priority to al-Qaeda and its affiliates; we are in a new reality."

The Richard Dimbleby lecture 2005, ’What Kind of Police Service Do We Want?, was to be broadcast on BBC1 tonight at 10.40pm.

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