Both the Metropolitan Police and the family of Jean Charles de Menezes were "disappointed" to hear that the force would be prosecuted under the Health and Safety at Work Act 1974 for not ensuring Jean Charles de Menezes's safety. Only the Met's disappointment is justified.

At first sight, it might seem strange to see Scotland Yard facing charges under legislation designed to make sure that employers provide a safe working environment for their staff.

In fact, there is a great deal to be said for using existing laws to satisfy the public's need for a trial that will examine how the entire Metropolitan police force operates - and to punish the force as a whole if its procedures are found wanting.

As the Crown Prosecution Service explained yesterday, there is no realistic prospect of charging police officers with the murder or manslaughter of Mr de Menezes.

The two officers who shot the Brazilian did so because they thought he had been identified to them as a suicide bomber. When taking action to defend others or to prevent crime, the law allows people to use such force as is reasonable in the circumstances. It makes no difference that Mr de Menezes was not a bomber at all: what matters are the circumstances as the officers believed them to be.

According to the CPS, there was evidence to support the officers' claim that they genuinely believed Mr de Menezes was a suicide bomber. Since prosecutors could not disprove that claim beyond reasonable doubt, they could not establish that the officers were guilty of murder or manslaughter.

Senior officers who "procured" the shooting would also have argued that they were acting to defend the public. The CPS decided that, while several people had made errors in planning and communication, no individual had been culpable to the degree necessary for a criminal offence.

It is hard to see how Mr de Menezes's family could successfully challenge the CPS's decision not to bring homicide charges. To be granted judicial review, they would have to show that a prosecutor with over 30 years' experience was acting unreasonably when he concluded that individual officers could not be convicted of unlawful killing.

That leaves Sir Ian Blair facing prosecution under the Health and Safety Act, for failing to ensure, "so far as is reasonably practicable", that members of the public who might be affected by his force "are not thereby exposed to risks to their health and safety".

Under section 40 of the 1974 Act, the onus is on the defendant to prove that it was not reasonably practicable for him to have done any more than in fact he did to satisfy his duties under the Act.

The police will have to establish that they took every precaution to ensure that an innocent person was not shot by mistake. That will surely require officers to give evidence about every stage in the police operation that led to Mr de Menezes's death, something they might otherwise prefer to keep confidential.

The jury will expect to hear about the skills and training of surveillance and firearms officers, as well as risk assessment techniques and the availability of necessary equipment.

During what may be a lengthy trial, they will have to outline their operational tactics in court. Officers will have to explain why surveillance officers lost sight of their suspect, why it took so long for the firearms unit to reach Stockwell station, and why Metropolitan Police radios could not be used underground on the British Transport Police network.

There have been heavy fines in other cases where members of the public have died as the result of corporate failures.

Last August, Transco was fined a record ?15 million over a gas explosion at a house in Larkhall, South Lanarkshire, that killed a couple and their two children in 1999.

Two months later, the rail maintenance firm Balfour Beatty was fined ?10 million - reduced on appeal to ?7.5 million - and Network Rail was fined ?3.5 million over the Hatfield train crash of 2000, in which four people died.

David Leckie, a solicitor with the law firm Maclay, Murray and Spens, said recent cases suggested that the Met could be fined millions of pounds if found guilty.

He added: "This will be a very wide-ranging and sweeping investigation into the whole system. It could be even more detailed than a manslaughter case."