Police have warned the mother of Sarah Payne that she may have been targeted by a detective working for the News of the World.
Sara Payne was given a phone shortly after her eight-year-old daughter, Sarah, was abducted and murdered in July 2000.
Police had previously told her that her name was not among those recorded by the NOTW investigator Glenn Mulcaire, but on Tuesday officers from Operation Weeting said they had found her personal details among the investigator's notes; the information had previously been thought to refer to a different target.
Friends of Ms Payne said she was "absolutely devastated and deeply disappointed" at the disclosure. Her campaign to give parents the right to know if paedophiles were living in their area was championed by the NOTW, in particular by its former editor Rebekah Brooks.
Ms Payne even wrote a farewell column for the paper's final edition, referring to its staff as "my good and trusted friends".
Last night, Ms Brooks put out a statement saying it was "unthinkable" that anyone on the newspaper knew that Ms Payne or others campaigning for Sarah's Law had been phone hacked. "The idea that anyone on the newspaper knew that Sara or the campaign team were targeted by Mr Mulcaire is unthinkable," she said. "It is imperative for Sara and the other victims of crime that these allegations are investigated and those culpable brought to justice."
But the Labour MP Chris Bryant – himself a victim of phone hacking – accused Ms Brooks of "utter hypocrisy". He said he was reminded of the quote from Hamlet that "one may smile, and smile, and be a villain".
The Labour MP Tom Watson said it was a "betrayal of trust". He said: "I think we need to know when Rebekah Brooks knew about this, and, having read the statement, I notice that she doesn't give a categorical denial, so I'd like to hear a bit more from Ms Brooks about what she knew.
"News International not only gave support to Sara Payne but they helped with the campaign. It now seems apparent that when they were doing all that and building a position of trust, their private investigator was targeting her."
News International said it was taking the allegations "very seriously" and is "deeply concerned, like everyone," adding it would fully co-operate with any criminal inquiry or civil proceedings.
Earlier this month, when it emerged that a phone belonging to the murdered teenager Milly Dowler had been hacked, rumours circulated among journalists that Ms Payne might also have been a victim. But police checked the name of Ms Payne against its database of all the information contained in the notebooks, computer records and audio tapes seized from Mulcaire in August 2006 and found nothing.
Ms Payne became a campaigner on child-abuse issues after her daughter was murdered by a paedophile in 2000. She is a member of the Phoenix Chief Advocates, which supports victims' families.
In the final edition of the NOTW, she wrote: "We have all seen the news this week and the terrible things that have happened, and I have no wish to sweep it under the carpet. Indeed, there were rumours – which turned out to be untrue – that I and my fellow Phoenix charity chiefs had our phones hacked. But today is a day to reflect, to look back and remember the passing of an old friend, the News of the World."
Since then, detectives from Operation Weeting have uncovered notes made by Mulcaire which include some of her details but had previously been thought to refer to a different target of his hacking.
The Phoenix Chief Advocates said in a statement: "Sara is absolutely devastated by this news; we're all deeply disappointed and are working to get her through it. Sara will continue to work with the proper authorities regarding this matter."
The news came as Lord Justice Leveson, the judge who will lead the inquiry into phone hacking, warned newspapers not to "close ranks" but help him expose journalistic malpractice. His inquiry will start in September.
Perhaps the most astonishing thing about Sara Payne's involvement in the phone-hacking scandal is that the News of the World courted her even after it became clear that the paper would have to close.
In the final edition of the paper, an article written by Mrs Payne, mother of the murdered schoolgirl Sarah, was given unrivalled prominence – a clear signal that the paper's original coverage of her story was considered the finest achievement in its long history.
There was another reason for highlighting the tragic figure of young Sarah in such a manner: the words of the murdered girl's mother amounted to hard evidence that the tabloid had, beneath everything else, a benevolent heart. "News of the World proved it is a force for good," said the headline.
Ms Payne talked of supportive relationships built with the paper's staff as it campaigned for the right of parents to know if they had a sex offender living nearby. Mulling the paper's closure, she wrote of "the passing of an old friend".
She noted "terrible things" had been uncovered, a reference to the hacking of the phone of Milly Dowler. "Indeed there were rumours – which turned out to be untrue – that I and my fellow Phoenix charity chiefs had our phones hacked," she wrote.
Nine days later, Rebekah Brooks, editor of the News of the World during the Sarah's Law campaign and when Milly's phone was hacked, gave evidence to MPs. "Part of the main focus of my editorship... was convincing Parliament that there needed to be radical changes to the Sex Offenders Act 1997 which came to be known as Sarah's law," she said.
"If I had a particular extra involvement in any... stories, then it would have been on the basis that I was trying to push and campaign for readers' rights on the 10 pieces of legislation that we got through on Sarah's Law."