The former girlfriend of Soham accused Mr Ian Huntley, Ms Maxine Carr, told his trial today she lied to protect him because she felt she was "doing the right thing".
She said Mr Huntley told her that Holly Wells and Jessica Chapman had been in her house on the day they vanished, but that they left alive.
Giving evidence for the first time at the Old Bailey, Ms Carr denied vehemently that she would have lied for Mr Huntley if she thought he had killed the 10-year-olds, who she had helped teach. She told his murder trial: "I just knew Ian, I knew he wouldn't have done anything like that."
Asked if she would have shielded him if he had told her, she said: "I would have been out of that house like a shot. Straight to the police or straight to the nearest person to tell them."
Ms Carr (26), a former teaching assistant in the girls' class, said she knew it was wrong to lie. But she added: "Even though I knew it wasn't right in moral terms, I thought I was doing the right thing at the time, for that person."
She insisted she had never heard the version of events Mr Huntley has put before the court - that Holly died accidentally in his bath and that he killed Jessica as he tried to silence her screams - until it was put before the jury in the last few days.
Mr Huntley (29), a former caretaker at Soham Village College, denies murdering the girls at the home he shared with Ms Carr but has admitted conspiring to pervert the course of justice.
He claims the girls died in his bathroom and that he bundled their lifeless bodies into his car and dumped them in the remote ditch near Lakenheath, Suffolk, where they were found 13 days later.
Ms Carr denies the conspiracy charge and two counts of assisting an offender. The prosecution alleges she gave him a false alibi by claiming she was in Soham on the day the girls died in August last year when she was really in Grimsby.