The ex-girlfriend of Babes in the Wood killer Russell Bishop lied "prolifically" in a bid to help clear him in his first trial, a court heard.
Jennifer Johnson, now 55, of Brighton, denies perverting the course of justice and perjury following the killings of two nine-year-old girls.
Bishop was cleared in 1987 of murdering Karen Hadaway and Nicola Fellows in a woodland den but was convicted in 2018.
Jurors heard Ms Johnson "significantly undermined" the 1987 trial.
Lewes Crown Court heard Ms Johnson told "lies that were important", including about whether or not a sweatshirt found near the scene belonged to Bishop.
Opening the prosecution's case, Alison Morgan QC said: "This defendant now accepts that she lied in both the witness statement and when she gave evidence on oath during the course of Russell Bishop's trial.
"By lying in the way that she did, it is the prosecution's case that she wanted to help Russell Bishop to be acquitted of those serious offences of murder."
She claimed Ms Johnson intended to pervert the course of justice and affect the outcome of the criminal trial.
Jurors were told Ms Johnson, of Saunders Park View, decided to "lie deliberately, in a sustained way and indeed prolifically", as well as trying to suggest police had acted improperly.
The case against Bishop was "significantly undermined as a result", Ms Morgan said.
The prosecutor said Ms Johnson's defence lawyers would suggest she was "acting under duress" and forced to make the witness statement and give false evidence by Bishop and his family.
She said: "She will say that she was young and that she was the victim of domestic abuse and coercive control."
But Ms Morgan said Ms Johnson had made a choice "to put herself and her family first" and "a choice to protect Russell Bishop".
The court has heard there is no doubt Ms Johnson and Bishop had a "volatile" and "violent" relationship.
The trial continues.