Baby Peter died after months of abuse despite visits from authorities
Baby Peter's mother has apologised for the "pain and suffering" endured by her son before his death.
In a letter to the judge who is due to sentence her alongside her boyfriend and lodger, she said: "I punish myself on a daily basis".
Peter died after months of abuse in Haringey, north London, in August 2007.
In a statement read at the Old Bailey, his father said he could not believe "such appalling cruelty" had been inflicted upon his son.
Peter's mother, her boyfriend and their lodger are to be sentenced for causing or allowing the baby's death.
Peter was 17 months old when he died with more than 50 injuries, despite being on Haringey Council's child protection register.
In a two-day sentencing hearing, Peter's father described him as a "cheerful, bubbly, smiling" boy.
"He was such an adorable and loveable little boy. He loved to be cuddled and tickled. He made anyone in his presence happy," he said.
Peter's father, who had separated from the boy's mother five months after Peter was born, said he could never come to terms with his death.
"I continue to feel extremely angry for what happened to Peter," he said.
On the last occasion he took Peter back to his mother, the boy started to scream, his father said.
He now realised that was because Baby Peter was going back to "a life of pain, fear and loneliness with no-one to care for or comfort him".
On 3 August 2007 he received a phone call from the boy's mother, saying Peter was on his way to hospital suffering breathing difficulties.
Baby Peter's father was "frantic with worry" and when he arrived at the hospital he was told his son had already been declared dead.
"I could not believe what was happening. I could not believe it was my son.
"He appeared to be asleep, and I just wanted to pick him up and take him home.
"But there was nothing I could do for him. I kissed him on his forehead and said goodbye. My son was gone forever."
Baby Peter's father said he comforted the boy's mother at that stage because he believed her explanation that he had died suddenly, probably from cot death.
Details of Peter's death emerged a week later.
His father said it was the "most horrendous time" and that he could not stop thinking about his son's last days.
"No human being, especially a child, deserves to suffer like Peter suffered in the last weeks and months of his life," he said.
His mother, her boyfriend, 32, and their lodger Jason Owens were found guilty of causing or allowing Peter's death.
The boyfriend will also be sentenced for raping a two-year-old girl.
Meanwhile three social workers who were sacked after Peter's death have appealed against their dismissals.
Haringey Council's deputy director of children, Cecilia Hitchen, social worker Maria Ward and team manager Gillie Christou were dismissed in April.