The claim that the News of the World deleted the voicemails of murdered schoolgirl Milly Dowler, giving her family false hope that she was alive, caused revulsion and outrage. When the story broke on 5 July, the Guardian left little doubt that reporters had deliberately deleted messages:

The messages were deleted by journalists in the first few days after Milly's disappearance in order to free up space for more messages. As a result friends and relatives of Milly concluded wrongly that she might still be alive. Police feared evidence may have been destroyed.

Now, however, new evidence has emerged which indicates that Milly's phone automatically deleted messages 72 hours after they were listened to. This means that while News of the World journalists may have inadvertently caused voicemails to be deleted, it was not deliberate, as the original report suggested.

Moreover, police have found that some messages were deleted before the News of the World began hacking Milly's phone. In a moving moment at the Leveson inquiry last month, Sally Dowler describe how the day after her daughter's disappearance, she had found that the voice mailbox had been emptied: "I just jumped and said 'She's picked up her voicemails, she's alive'." According to the police evidence, this took place on 24 March 2002. Police now believe that this could not have been caused by News of the World, which had not yet instructed the private investigator Glenn Mulcaire to hack Milly's phone. It is still unclear what could have caused this deletion.

The Guardian's report on the new revelation quotes the Dowlers' lawyer Mark Lewis reiterating that the missing girl's voicemail was still hacked:

It remains unchallenged that the News of the World listened to Milly Dowler's voicemail and eavesdropped on deeply personal messages which were being left for her by her distraught friends and family.

Fundamentally, it is true that wrong is wrong, and that her voicemails should never have been accessed. It is also worth pointing out that the claim about the voicemails being deleted is by no means the only reason the paper was shut: that Milly's voicemails were hacked at all took disgust at phone-hacking to another level, while Rebekah Brooks told staff that even this was not the only reason for the closure.

However -- as the outraged reaction on Twitter has shown -- this is embarrassing for the Guardian, given that it was reporting on the flaws of another paper. It is a lesson in making sure all the facts are watertight before making unequivocal assertions.

UPDATE 12.20pm: David Leigh, the Guardian's investigations editor, has tweeted: "Guardian was first with #Dowler deletion story - and first w story when Dowler police changed their minds. That's good journalism".