The lies and deceit of Ian Huntley and Maxine Carr do not prove their guilt, the Soham trial judge warned today.
Their continued lies were "inevitable" once Mr Huntley started trying to hide the fact that the girls died in his home and once Ms Carr agreed to give him a fake alibi, Mr Justice Moses said.
He told the jury that they should only consider their deceit as proof of the case against them if they believed it was done to hide their guilt over the deaths of Holly Wells and Jessica Chapman.
Speaking about Mr Huntley, the judge said: "You can only act on any lies he told or any aspect of his activities if you are sure the only explanation for them was that he was lying about and trying to cover up murder.
"In other words, if those actions or those lies went to prove not only that his account was untrue but that he intended to kill or cause really serious harm. And those lies and actions have no other explanation."
Mr Justice Moses told the jury that they must take the same approach in relation to Ms Carr, who denies conspiring to pervert the course of justice and two charges of assisting an offender.
He said the fact that she lied and concealed knowledge that the girls were in the house was not necessarily evidence that she knew or believed Mr Huntley was guilty of manslaughter or murder.
He warned: "You must be sure that she, in fact, accepted that Ian Huntley had murdered the girl referred to in the count under consideration or was guilty of manslaughter. But the mere fact that she shut her eyes to what you regard as the obvious is not enough."
Earlier, the judge instructed jurors to reach their verdicts on the evidence alone and "uninfluenced" by emotion.
Mr Justice Moses, summing up on the 26th day of the Old Bailey trial, said that it was "idle to pretend" the case did not provoke an emotional response. But he advised the jurors to concentrate on whether Ian Huntley was telling the truth about how the schoolgirls died when they retired to consider their verdict tomorrow.
He also informed them that Mr Huntley could be found guilty of murdering the girls even if no motive, sexual or otherwise, had been established.
He said: "The defence argues, through Mr [Stephen] Coward, that you cannot prove murder if you cannot prove a sexual motive in this case. Well, ladies and gentlemen, that is not a rule. It is an argument for you to consider.
"Certainly, if you take the view there was no motive proved, or there may not be a motive, it is a factor you must take into account.
"But, if the evidence makes you sure that he was guilty, the absence of any motives is no defence, and you may feel ... it is far better to concentrate on what the defendant says happened in the bathroom and decide whether that was the truth or may have been the truth."
Mr Huntley, 29, has admitted that Holly died accidentally in his bath and that he killed Jessica as he tried to silence her screams. He insists that he did not mean to kill her.
On the first count, alleging murder of Jessica, the judge said: "If you are sure Ian Huntley was lying to you about how Jessica Chapman died, what are you left with?
"She was a healthy, happy and fit girl when she went into the house, and she died there.
"If you are left with the fact he has lied to you about how she died, on the basis of those facts you will be entitled to conclude the reason he has lied to you is because he had murdered her." Mr Justice Moses said that the jury should consider the evidence of the pathologist about how long it would have taken Jessica to die, and what Mr Huntley's intent was during that time.
He told them: "If you're not sure of that murderous intention, then he's guilty of the lesser offence of manslaughter."
The judge said that, if the jury believed Mr Huntley's account of how Holly died was the truth, or could be the truth, they would have to find him not guilty of murder or manslaughter.
However, he added: "If you believe the defendant or think that he may have been telling the truth, you would be entitled to conclude that he is guilty of manslaughter by gross negligence."
The judge also warned the jurors not to be influenced by "emotional reactions" to the evidence before them.
He said that Mr Huntley's description "of how he came to dispose of their bodies may have appalled you, but remember, emotion can play no part in your deliberations".
Mr Justice Moses said the jury would need to consider that Mr Huntley was a man of previous good character, in that he had never been accused of a similar offence before. He added: "It's not a defence, but it's a factor."
He also said they must ignore the fact that he was previously accused of rape, as the case was later dropped at court. He said it would be "wholly unfair to say in light of what we know that perhaps there might have been something in it".
Mr Huntley, a former caretaker at Soham Village College, denies murdering the two 10-year-olds on Sunday August 4 last year, but has admitted a single charge of conspiring to pervert the course of justice.
In its closing statement yesterday, his legal team conceded there was evidence that he was guilty of manslaughter and was "worthy of punishment". However, they said the jury had "waited in vain" for evidence that he was guilty of murder.
