Ian Huntley has said he is responsible for the deaths of Jessica Chapman and Holly Wells, but denies their murder.
Mr Huntley told the court how the girls died
He told the Old Bailey the 10-year-olds died accidentally in his home in Soham, Cambridgeshire, on 4 August 2002, before he dumped their bodies.
On the verge of breaking down, he expressed regret and said: "I accept I am responsible for the deaths."
Mr Huntley, 29, denies murder and admits one charge of conspiring to pervert the course of justice.
His former girlfriend Maxine Carr denies helping an offender and conspiring to pervert the course of justice.
Mr Huntley, first witness for the defence, said he had never intended to kill the 10-year-olds or to cause
Trying to help Holly to stem a nosebleed, he invited the two friends in to his house and up to his bathroom, he said.
Mr Huntley claims Holly accidentally drowned in his bathtub, and that he apparently smothered Jessica while trying to stop her screams.
He told the court: "I wish I could turn back the clock, I wish I could do things
differently, I just wish none of this had ever happened.
"I'm sorry for what's happened and I'm ashamed of what I did.
"I accept that I'm responsible for the deaths of Holly and Jessica but
there's nothing I can do about it now. I sincerely wish there was."
But asked by his barrister Stephen Coward QC if he had intended to kill them, he said: "No I didn't."
The accidents were so freakish that he did not call the police for fear of not being believed, he said.
And his voice cracked as he told the court how frightened and ashamed he
He became depressed and attempted suicide while in prison awaiting trial, the court heard.
But he promised his parents he would go to trial and tell the girls' parents what happened.
Mr Huntley said he had wanted to tell Ms Carr the truth about what happened "several times" but never had.
All she knew was the girls came to the house and then walked away again, he said.
"This is the first time that Maxine would have heard what happened that day," he told the court.
He denied he persuaded Ms Carr to tell police she had been with him in Soham when in fact she had been in Grimsby.
Mr Huntley was in the witness box for the first time on Monday and spent a total of three-and-a-half hours giving evidence in a faltering, hushed voice.
He said he could not remember how the girls died until after his attempted
overdose in Woodhill Prison in June this year.
Earlier in the day, the jury heard how after the girls died in the bathroom, Mr Huntley, who had checked the girls for pulse and breathing, sat on the landing but admitted he had no memory of how he got there.
He then put the bodies in the boot of his car and he drove "with no destination in mind".
He packed two bin liners and a petrol can "to destroy any evidence" and stopped the car down a remote lane near Lakenheath called Common Drive.
As he put the bodies in a ditch, he wore the bin liners to cover his trainers.
Then he set light to them and took the clothes back to the school, where he burned them in a bin, he said.
He questioned police about the search in order to cover his tracks, he admitted.
The case continues.