Leading article: This inquiry looks less substantial by the day

The last day of the Leveson Inquiry before the Christmas recess threw up two developments with the potential to be game-changers. First, on the basis that he overheard show-business journalists talking about the practice openly, a former reporter at the Daily Mirror testified that phone-hacking appeared to be a "bog-standard journalistic tool" for obtaining information at the paper. Second, and outside the inquiry proper, The Guardian newspaper printed an extensive clarification, accepting that journalists from the News of the World probably did not delete messages from the mobile phone of Milly Dowler after she had been murdered.

The testimony of the Mirror journalist, James Hipwell, broke new ground, not least because it concerned a newspaper outside Rupert Murdoch's News International. Here was first-hand evidence that phone-hacking was regarded as nothing out of the ordinary and was not unique to the Murdoch group. It will be hard from now on for the inquiry to be dismissed as just a stick for beating News International. There are more individuals and organisations in the dock.

Mr Hipwell, who served a prison sentence five years ago for writing about companies whose shares he owned, was also forthright on the subject of how far up the Mirror's hierarchy the knowledge of phone-hacking probably went. Testifying just a day after his then editor, Piers Morgan, had denied any awareness of illegal practice at the paper, he said that in his view it was "very unlikely" that Mr Morgan did not know, given his "very hands-on" approach and his particular interest in the showbiz beat.

In this respect, Mr Hipwell's evidence conformed to an emerging pattern, according to which reporters and private investigators, apparently caught out, say they believed the practice had official sanction, while editors and executives profess ignorance. Yesterday's evidence conveyed how improbable such a cut-off actually is. Only last week, James Murdoch, the executive chairman of News International, conceded that he had received an emailed briefing on hacking, but that he had not read it properly.

Even as danger seems to be lapping around the upper reaches of the popular press, however, the rationale for instituting the Leveson Inquiry in the first place suddenly looks shaky. It was The Guardian's claim about NOTW journalists deleting Milly's messages that revived public indignation about phone hacking and convinced politicians they had to "do something". That something became the inquiry headed by Lord Leveson.

It was not just the politicians who were spurred into action. When he learnt of the distress to the Dowler family – who had interpreted the deleted voicemails as evidence that Milly was alive – Rupert Murdoch made public and personal apologies to her parents, agreed a £3m payment, and summarily closed the NOTW. Yet that report, as The Guardian now admits, was based on assumption, not fact. While journalists indeed hacked the messages, the deletions were probably made automatically.

The "what ifs" that follow are legion. What if The Guardian had reported only the hacking and not the deleting, would the public outcry have been as great? Would the Prime Minister have felt compelled to respond? Would there have been an inquiry at all, on top of the police investigation already in train? After all, if phone-hacking was going on – which it was – it was a breach not only of ethics, but the law.

One conclusion might be that the Leveson Inquiry is doing the right thing, even if it was set up for the wrong reason. But this would be to make the best of what is, at root, a bad job. The question must be faced squarely: is it right that this inquiry, which could transform regulation of the British press, should proceed at all, now it is clear that it was built on a misapprehension?