Updated 14:36, 13 May 2013: witness impact statements added; 15:15 sentencing date
In dramatic scenes at the Old Bailey, the 37-year-old window cleaner hung his head in the dock as he admitted he killed the teenager.
Lord Carlile, representing Hazell, said his client wanted to make it known that "Tia's family have suffered enough and he did not want to put them through any further stages of this trial or this process".
Hazell, wearing a washed-out grey T-shirt, leaned forward as Lord Carlile asked for the charge to be put again.He clutched his hands in front of him and there was a slight pause when he was asked to plead. And then he said "guilty" as gasps from the public gallery rang around the oak-lined courtroom.
Hazell has finally admitted his guilt
Tia's father, Steve Carter, broke down in tears as Hazell changed his plea. The development came after four days of graphic evidence during which Tia's mother, Natalie Sharp, frequently had to leave the courtroom, visibly distressed.
The prosecution case included a grotesque photograph of a girl alleged to be Tia after she died. In the shot, a naked girl, whose face cannot be seen, is posed on all fours on a bed. Mr Justice Nicol is due to sentence Hazell later today.
Last week, the court heard how Hazell wrote to his father from prison claiming that her death was an accident and begging for forgiveness. He said he had made "one mistake and my whole world has collapsed".
The letter, read to jurors at the Old Bailey on Friday, was topped with the words "forgive me" and a sad face.
Hazell was said by prosecutors to have sexually assaulted the 12-year-old then killed her and put her body in the loft of the home that he shared with her grandmother in south London.
Tia Sharp was murdered in August last year
He had denied the charge, claiming that Tia died in an accident and he panicked and hid her body.
Hazell wrote: "What happened I will explain in time, but put it this way, it was an accident and I panicked. Stupid I know but for my stupidity I'm looking at 15 to 18 years. I regret it every second of every day.
"I think about ending my life because if I don't someone will, that's definite."
Im a witness impact statement, Tia's mother accused Hazell of breaking "the ultimate trust" she had given him when he brutally murdered the schoolgirl.
"I gave the ultimate trust to Stuart, there is so much I want to ask him. Sometimes I want to pity him, sometimes I want to hurt him. But I can't hurt him like he hurt me."
Ms Sharp said she would like to meet Hazell and make him answer her questions, but she doubted that would be allowed.
She wrote about how Tia's birth had changed her life for the better, giving her "a reason to keep out of trouble".
"Since Tia was taken, I have lost my trust in everyone," she said in the statement.
"It is too hard for me to believe that she is really gone. I try to think of her on a sleepover at a friend's house.
"Jack, my eldest son, who is three, asked me just this week if Tia was coming home from school soon. I've had to tell him the truth. It really made him cry.
"I told him that Tia is a star in the sky and now, when we go up to say goodnight, we look out of the bedroom window and speak to the star, the one that was bought in Tia's name.
"I breathe for my children. I fear anyone hurting my boys. I fear that if anyone touches my sons or does anything to them, what I might do, I am so scared and angry."
Ms Sharp also hit out at people who criticised her parenting via social media, saying: "My close family are closer still.
"My brother is struggling to work, my mum was suspended from her job when Stuart was arrested.
"I've heard people saying Tia was mistreated and that she's lucky to be away from this world.
"My Tia wanted for nothing. I have gone without so many times to give my children everything and the truth is that strangers have said things that have been repeated in the press and social media that have been unbelievable."
Hazell sat in the dock with his head bowed as the court heard Ms Sharp's victim impact statement and one from Tia's father, Steven Carter.
In it, Mr Carter said his daughter's murder had "shattered mine and my family's hearts" and robbed them of the chance to see her grow into a woman with a family of her own.
It has also left the family unwilling to let their children go out to play in case something happened to them, the court heard.
"My daughter Tia's life has been taken from us all, as we will never get the opportunity to share her 13th, 16th, 18th or 21st birthdays," he said.
"We will never have the chance to see Tia walk down the aisle and get married, and have children of her own. We have all lost someone special.
"The love for Tia will always be with us, our memories of Tia smiling and playing will never be forgotten.
"My last memory of Tia is her jumping into my arms, giving me a kiss and her telling me she loved me.
"I will never get this opportunity again, but it is one of the many memories I will cherish for the rest of my life."
He added that he and other members of his family had suffered from post-traumatic stress disorder (PTSD) since Tia's murder.
Hazell will be sentenced on Tuesday.
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