Head injuries alone are not likely to be enough to charge someone with homicide, attempted murder or assault in cases of so-called shaken baby syndrome, the Crown Prosecution Service (CPS) said today.
The CPS also urged prosecutors to challenge defence experts who claim the three head injuries generally associated with the syndrome may be explained by a lack of oxygen, infection, or raised intracranial pressure.
The guidance updates that issued five years ago following concerns over the evidence of the paediatrician Professor Sir Roy Meadow in the Angela Cannings prosecution and other high profile cases.
Karen Squibb-Williams, the senior policy adviser in the CPS's strategy and policy directorate, said: "These are complex and sensitive cases.
"The guidance makes clear that it is unlikely that a charge for a homicide or attempted murder or assault offence could be justified where the only evidence available is the triad of injuries."
In cases where the three internal head injuries are found, "the prosecutor will always consider all the surrounding circumstances and the evidence in each case before reaching a decision", she said.
Ms Squibb-Williams added that prosecutors should strongly resist defence claims that the three specific injuries - bleeding into the linings of the eyes, bleeding beneath the dural membrane of the brain, and damage to the brain affecting function - may be explained by a lack of oxygen, infection, or raised intracranial pressure.
The defence experts' theory, known as 'the unified hypothesis', was rejected by the Court of Appeal as recently as July, she said.
She went on: "The updated guidance makes clear prosecutors should continue to resist defence challenges to the established theory that non accidental head injury (Nahi) cases will usually be diagnosed in children where sufficient force has been used to produce a combination of three internal head injuries, known as 'a triad of intracranial injuries'.
"To prove a Nahi case you will usually require the triad of injuries plus supporting evidence."
Ms Squibb-Williams added: "Each case will have its own individual facts and very careful consideration will be given in deciding whether there is sufficient evidence for a realistic prospect of conviction, and then in considering whether it is in the public interest to bring a prosecution."
Mrs Cannings was found guilty of smothering her seven-week-old son in 1991 and her 18-week-old son in 1999.
But the judges who released her ruled that a prosecution should not be brought when it rested "exclusively" on a serious disagreement between distinguished experts.
Sir Roy has since been found guilty of serious professional misconduct and struck off the medical register.
He also gave evidence in the case of Donna Anthony who was imprisoned for life in 1998 for killing her 11-month-old daughter and four-month-old son. She was also freed on appeal in April 2005.
Mrs Anthony's case was one of the 28 referred to the Criminal Cases Review Commission after Mrs Cannings was released.
Today's guidance updates that issued following Mrs Cannings' case, which involved sudden infant death syndrome (Sids).
It also follows the case of a young mother accused of shaking her eight-month-old son to death who faced two trials before a judge threw out the manslaughter charge against her.
Fatima Miah denied prosecution claims that she had shaken baby Anas in a fit of temper in May 2007, leaving him with the brain injury from which he died, telling police he collapsed after falling off the sofa.
Judge Timothy Pontius ordered jurors at the Old Bailey formally to clear her in July 2009 after he was asked to make a ruling on the conflicting evidence of medical experts about the cause of the child's death.
On the day Anas died, an ambulance was called to the flat and he was found on the floor, not breathing, and his heart stopped for 40 minutes.
Miah, of White City, west London, faced her first trial at the Old Bailey in November 2008 but the murder charge was thrown out by a judge and jurors were unable to agree their verdict on the charge of manslaughter.
The mother faced a second trial but the judge decided to throw it out after hearing both the prosecution and the defence case.
The triad of internal head injuries was at the heart of the case, but the judge said there was a "fundamental conflict of expert opinion" on the cause of death and no "clear evidence" to back one side or the other, so he would have to direct the jury to enter a not guilty verdict.
There was no evidence of external injuries and the mother had no previous convictions or history of abuse.