A Crown Prosecution Service officer broke down as she testified she never had sex with a detective who had been accused of downloading pornography.
Brian Stevens and Louise Austin deny the charges against them
Louise Austin, 32, is standing trial with Brian Stevens, a detective who acted as a liaison officer to the family of murdered Jessica Chapman.
She told the Old Bailey that she did not concoct an alibi to help her friend. The case was later dropped.
Both deny conspiring to pervert the course of justice.
Miss Austin sobbed for much of her evidence, particularly when pressed over her previous relationships.
Asked by her defence barrister Christopher Sallon QC, whether her relationship with Mr Stevens, 43, had been "sexual", Ms Austin replied: "No it wasn't."
She told the court that Mr Stevens had helped her through difficult times in her
personal life, and that he did stay with her in June 2002, immediately after her then
boyfriend ended their relationship.
But she said she could not be exactly sure when the crucial visit happened.
Mr Sallon asked: "Were you deliberately and falsely giving misleading
information in order to provide a false alibi?"
The witness replied: "No, I wasn't."
'I was horrified'
She said she was supportive when her friend was arrested for alleged internet offences in September 2002.
"His name was on the news," she said. "I was horrified. I went straight round to his house to see (his family) and offered them all the support I could give them."
Mr Stevens, liaison officer to the Soham family for six weeks, was suspended after he was alleged to have downloaded pornographic material on his laptop computer.
Prosecutors offered no evidence against the Cambridgeshire detective at Snaresbrook Crown Court in London in August 2003 over the alleged computer offences.
But he was later arrested again and accused, along with Miss Austin, an executive case officer for the CPS, of concocting a false alibi for the times when the images were alleged to have been downloaded.
The defendants had both given statements to police in which they said he was staying at her house without his laptop when the images were said to have been downloaded on 9 June, 2002.