Rebekah Brooks has said she first heard of the Milly Dowler hacking claims two weeks ago when she read about them in the press.
Brooks, who was editor of the News of the World when the newspaper allegedly hacked into the murdered schoolgirl's voicemails in 2002, has denied any knowledge of the allegations.
Mrs Brooks spoke of when she initially heard about the hacking of Milly Dowler's phone: "My instant reaction, like everybody else, was one of shock and disgust."
Speaking at the Commons Culture committee today, Mrs Brooks said that for ""a family who had suffered so much already, these allegations clearly added immeasurably to their suffering".
"The first thing I did was write to Mr and Mrs Dowler with a full apology to say that we would get to the bottom of the allegations."
She added: "The idea that Milly Dowler's phone was accessed by someone being paid by the News of the World, or even worse authorised by someone at the News of the World, is as abhorrent to me as it is to everyone in this room.
"And it is an ultimate regret that the speed in which we have tried to find out the bottom of these investigations has been too slow."
Mrs Brooks, who resigned from her position as News International chief executive last week, was arrested by police on Sunday. She was later released on bail.
Mrs Brooks did admit that the NOTW "used private detecties like most papers on Fleet Street".
She said: "I think in the main, my use of private investigators while I was editor of the News of the World was purely legitimate.
"And in pursuit in the main, as you know, for the addresses and whereabouts of convicted paedophiles through Sarah's Law.
"And that is my majority use, if not almost exclusive use, of private investigators myself."
But Mrs Brooks said she didn't know that Glenn Mulcaire was one of the detectives being used by NOTW. Mulcaire and ex-NOTW royal reporter Clive Goodman were jailed in 2007 over phone hacking.
The decision to close the NOTW two weeks ago was taken because the paper had lost the trust of its readers, said Mrs Brooks.
""Once that trust was broken, we felt that that was the right decision. Of course, it wasn't the right decision for the hundreds of journalists who worked on there, had done nothing wrong, were in no way responsible," she said.
"Every single one of them will be offered a job."
In 2003, Mrs Brooks admitted that payments had been made to the police in the past.
Referring to her remarks, she said: "I have never paid a policeman myself. I have never sanctioned, or knowingly sanctioned, a payment to a police officer."
