The BBC's Richard Bilton reports on the new research

Doctors at the University of Birmingham illustrated their findings by using an egg - representing the brain - suspended in a jar filled with water, which represented the baby's skull and cerebro-spinal fluid.

They found that, unless the jar was struck hard enough to break the glass, the force of the blow could not make the egg touch the sides.

However, when the jar was shaken, even lightly, the egg hit the sides.

Dr Sunderland: "Never shake a baby"

It is the force with which the brain strikes the skull which determines whether brain damage sufficient to cause permanent damage or death happens.

Dr Robert Sunderland, a consultant paediatrician at the university, said: "Hit the jar as hard as you like - the egg will not touch the edge of the jar unless you break the glass.

"But give the jar just a flick of the wrist and the egg will crack against the side."

He said that previous studies had underestimated the cushioning qualities of cerebro-spinal fluid.

David Hinchliffe MP, chairman of the Commons Health Select Committee, said the Government should now act.

The BBC's Richard Bilton: "As little as two or three shakes may give a baby brain damage"

He said: "The government should now look at whether it is appropriate to issue new guidelines to those involved in child protection."

The researchers' own figures showed it was 30 times easier to kill by shaking than hitting.

The importance of never shaking babies, regardless of the force involved, has been emphasised by child safety organisations, particularly in the light of recent cases in which nannies have been found to have harmed children by shaking them.

Three court cases

Louise Woodward, from Cheshire, was convicted of manslaughter in a US court following the death of toddler Matthew Eappen.

Also in the US, British nanny Manjit Basuta was found guilty of fatal child abuse after a child in her care died.

Nanny Louise Woodward was convicted of manslaughter

And an Australian nanny, Louise Sullivan, who pleaded guilty to manslaughter after the death of a six-month-old child she shook, was given a suspended sentence by a British court.

Dr Sunderland said that his research reinforced the "never shake" message.

"When your baby cries, if you feel anger, walk away. At least you know it's alive. Never deal with your anger while the baby is in your hands."

When the brain strikes the skull cavity, it can bruise, swell and haemorrhage, sometimes causing permanent brain damage or blindness.