Ms Brooks, while editor of NOTW, used Steve Whittamore, a private detective who specialised in obtaining illegal information, to "convert" a mobile phone number to find its registered owner. Mr Whittamore also provided the paper with the Dowlers' ex-directory home phone number.
The Information Commissioner's Office, which successfully prosecuted Whittamore for breaches of the Data Protection Act in 2005, said last night it would have been illegal to obtain the mobile conversion if the details had been "blagged" from a phone company.
Ms Brooks, who said yesterday she was "shocked and appalled" at the latest hacking claims, admitted requesting the information. But she said it could be obtained by "perfectly legitimate means". She faced demands for her resignation last night.
The revelation came as News International battled a political and commercial firestorm over the disclosure that its bestselling paper interfered with the police investigation into Milly's disappearance in March 2002 by hacking into her mobile phone and deleting messages. One big advertiser, Ford, announced it was suspending its account with the paper while the energy company Npower and Halifax bank said they were considering options. Thousands of readers joined boycott campaigns on Facebook and Twitter.
An emergency three-hour debate is to be held in the House of Commons today. The Labour leader Ed Miliband hardened his position on the scandal, demanding a public inquiry and calling for Ms Brooks to "consider her conscience and consider her position".
David Cameron described the hacking as "quite shocking" and a "truly dreadful act", but rebuffed the call for a public inquiry. He insisted Scotland Yard be allowed to follow the evidence wherever it led.
Yesterday it led to Cambridgeshire, where police confirmed there was evidence that Glenn Mulcaire, the private investigator who is accused of hacking Milly's phone, targeted the families of Holly Wells and Jessica Chapman, who were murdered in Soham in 2002.
Responding to growing clamour for her to step down, Ms Brooks yesterday told News International staff it was "inconceivable" that she knew of or sanctioned the hacking of Milly's mobile phone. In a passionate defence of her position, she wrote: "I have to tell you that I am sickened that these events are alleged to have happened. Not just because I was editor of the News of the World at the time, but if the accusations are true, the devastating effect on Milly Dowler's family is unforgivable."
No evidence has been presented that Ms Brooks was aware of Mulcaire's activities surrounding Milly's disappearance. But an investigation by The Independent shows she was aware of the existence of Whittamore, who used an associate to obtain the Dowlers' home phone number from BT, and made use of his services in an unrelated case.
A copy of the "Blue Book" obtained by The Independent, which covers more than 1,000 transactions carried out for New International's titles between 2000 and 2003, records a request in 2001 from Ms Brooks (whose surname was then Wade) for a "mobile conversion" along with a mobile phone number. She made a second request for an electoral roll search for an address in Doncaster, South Yorkshire. The address was occupied at the time by a painter-decorator who lived in a flat above a bike shop in the town.
A friend said: "I have no idea why the editor of the News of the World would have been interested in him. He's just an ordinary guy."
When asked last year by MPs to explain the circumstances around her request, Ms Brooks said she could no longer remember why she wanted to convert the number.
"This was nine years ago and I cannot recall why I required this particular conversion," she wrote.
"You should note that 'conversion'... is often carried out through perfectly legitimate means such as a web search."
A spokeswoman for the Information Commission Office said: "If that information was obtained by 'blagging' then it would have been illegal under Section 55 of the Data Protection Act."
As the fallout from the row continued, Mulcaire, whose home was besieged by reporters, said he had been acting under "relentless pressure" from the paper. "Working for the News of the World was never easy," he said in a statement.
"There was relentless pressure. There was a constant demand for results. I knew what we did pushed the limits ethically. But, at the time, I didn't understand that I had broken the law at all. I never had any intention of interfering with any police inquiry into any crime."
News International last night failed to respond to a request for comment on Ms Brooks' requests to Whittamore. Simon Greenberg, a spokesman for the company, said that Ms Brooks would not be stepping down over the hacking of Milly's phone: "This happened back in 2002, she's now chief executive of a company in 2011. She's absolutely determined to get to the bottom of this issue."
He said the company had launched a full inquiry to establish the facts of the hacking of Milly's phone during Ms Brooks' time as editor.
It emerged last night that NI had passed internal emails to Scotland Yard suggesting that payments were made to police by NOTW staff during the editorship of Andy Coulson between 2003 and 2007. The BBC reported that the documents appear to show that Mr Coulson, former spokesman of David Cameron, approved the alleged payments.
A source at Ofcom said it was considering applying the "fit and proper persons" test to the planned takeover of BSkyB by Murdoch's News Corp once the hacking investigation was complete.
Mulcaire's apology: 'There was relentless pressure on me'
Glenn Mulcaire, the News of the World private investigator who is accused of hacking Milly Dowler's phone, issued an apology yesterday to his victims, explaining his actions as the result of his ignorance of the law and the pressure placed upon him by the Sunday newspaper.
"I want to apologise to anybody who was hurt or upset by what I have done," he said. "I've been to court. I've pleaded guilty. And I've gone to prison and been punished. I still face the possibility of further criminal prosecution."
He avoided any direct mention of Milly, but said he was reacting to the news that has emerged in recent days.
"Working for the News of the World was never easy," he said. "There was relentless pressure. There was a constant demand for results. I knew what we did pushed the limits ethically. But, at the time, I didn't understand that I had broken the law at all. I never had any intention of interfering with any police inquiry into any crime."
As a crowd of reporters surrounded his home, he appealed for the privacy of his family to be respected, adding that he knew he had "brought the vilification I am experiencing upon myself" but that his family could not be blamed.
He also offered the defence that "sometimes what I did was for what I thought was the greater good, to carry out investigative journalism".
