Ian Huntley said he "panicked and froze" after Holly Wells fell into his bath and Jessica Chapman started to scream in his bathroom.
Mr Huntley told the court of events on the day the girls disappeared
He told the Old Bailey the girls died in accidents in his house on the day they disappeared in August 2002.
The 29-year-old said he wanted to turn back the clock and added: "I accept I am responsible for the deaths."
Mr Huntley 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 told the court: "I accept I am responsible for the deaths of Holly and Jessica...but there is nothing I can do about it now."
He said he did not call 999 after Holly Wells and Jessica
Chapman died in his house because he did not think the police would have believed what happened.
He claims the girls died in a series of accidents at his home after Holly suffered a nosebleed.
She accidentally fell into a bath half full of water, while Jessica died after he tried to stop her screaming by putting a hand over her mouth, he said.
He placed their bodies into his car and drove away, with two black bin liners and a petrol can, the court heard.
'No sexual assault'
The petrol was to "destroy any evidence", including dog hairs and carpet fibres, Mr Huntley said.
After arriving at a spot in Lakenheath in Suffolk, near where his father used to live, he stopped the car after seeing a "density of vegetation", where he rolled their bodies into a ditch.
He then removed their clothes but denied touching them in a sexual way.
After spreading the petrol, he lit it and drove back to Soham where he burned the clothes in a yellow bin on the school premises.
Describing his feeling afterwards, he said: "I tried on several occasions to tell Maxine... couldn't get the words out.
"How do you tell somebody that you are responsible for the death of two girls and then what happened after that?"
He added: "I was frightened and ashamed of what I had done."
On the following day, he changed his car tyres and rang Maxine Carr but did not tell her what happened, the court heard.
Mr Huntley said he never told Ms Carr the truth and "there was no way she thought I was in any way responsible".
He was contemplating taking a drugs overdose because "it's been very difficult to live with what's happened."
At the time of his arrest on 17 August, the two defendants had not "firmly" agreed on a story to give him an alibi, Mr Huntley said.
Earlier on Monday, Mr Huntley described the events leading to the girls' deaths.
He said he was preparing to wash his dog, Sadie, when the girls, wearing Manchester United tops, had walked up and asked about Ms Carr, their classroom assistant.
During the conversation outside Mr Huntley's house, he said he gave Holly some tissue for a nose bleed and suggested cold water might help stop it.
All three went upstairs to the bathroom, where Mr Huntley said a bath, prepared to wash the dog, was under half full - "probably 6-8 inches of water".
In the bathroom, Mr Huntley told the court, he passed Holly some tissue and slipped. although he was unsure if there was physical contact with Holly.
Mr Huntley said after Holly had fallen into the bath, she did not move and "I just panicked and froze."
He denied deliberately pushing Holly into the bath, but said "that is a possibility".
Jessica was "screaming and shouting at me 'You pushed her' over and over", so he covered her mouth with one hand.
When he removed his hand, Jessica fell on the floor and neither girl had a pulse, but he
did not attempt resuscitation.
The case continues.