If anyone still believed that the phone hacking scandal was “just” about celebrities, the allegation that the News of the World hacked Milly Dowler’s voicemails must lay the idea to rest.

No matter how ordinary and vulnerable you were, no matter how tragic your circumstances — in this case it was a missing, murdered Surrey schoolgirl — on this evidence you were a potential target for the Rupert Murdoch’s Sunday tabloid.

And if anyone besides News International and its friends and allies had any doubt that we needed a public inquiry to get to the bottom of this affair, surely this will have convinced them. In this instance alone we not only need to know the facts of what happened — of who did it, who ordered it and who knew about it — but we must also find out why it has taken until now for this to become public.

For the Guardian is saying that the key evidence was in the bin bags of material seized from private investigator Glenn Mulcaire in August 2006. In other words, the police have had this material for five whole years. Either they knew about it and ignored it — which would suggest either stupidity or corruption of a kind that is hard even to contemplate — or they have only just found it. That would be less astonishing, but not much less worrying.

And if the police conduct is a matter for public concern, what of the company, which covered up this scandal for years, telling us that only one reporter was involved and that it had investigated itself thoroughly — and which is even now as grudging as it can be with information and evidence?

But the case for a public inquiry has become urgent for other reasons than these, because in recent weeks there has been every sign that without one the scandal will be killed off by the year’s end. The civil litigants — the victims of hacking who have sued — are settling, one by one. Often they have no choice because the courts would punish them for holding out. And the criminal prosecutions — if they come — may well be much more peremptory affairs than many expected.

Lawyers following these cases warn that every person charged may plead guilty, just as Mulcaire and royal reporter Clive Goodman did back in 2007. That would mean there would be no trials, just short agreed narratives and brief sentencing hearings. Nothing about the wider issues would come out.

So if you have any interest in knowing the truth about the hacking scandal — and the Dowler allegations demonstrate vividly that we all have such an interest, no matter how innocent and ordinary we may be — then your only hope is a public inquiry.

Here I will declare an interest of my own. For the past few weeks I have been working with others, notably the Media Standards Trust, to set up a public campaign, called Hacked Off, to demand an inquiry.

We are not quite ready, I confess — it will be launched on Wednesday and until then the website http://hackinginquiry.org is only a holding page. We will have a manifesto, a petition, dozens of distinguished supporters and soon a programme of public events, but what we will need most of all is your support. Please go to the site on Wednesday or soon afterwards.

The vested interests here are tremendously powerful and winning this inquiry, even after these latest allegations, is not like to be easy.

Brian Cathcart teaches journalism at Kingston University London. He tweets at @BrianCathcart