Dr Leach said that babies should not be left to cry for more than 30 minutes, as recent scientific tests have shown increased levels of the stress hormone cortisol in distressed infants.
Over long periods this can be toxic for the developing brain, Dr Leach said in her book The Essential First Year - What Parents Need to Know, published this month.
In an interview with The Independent, Dr Leach said, 'It is not an opinion but a fact that it's potentially damaging to leave babies to cry. Now we know that, why risk it?'
She added, 'A baby who is left crying for long enough will eventually stop, but not because he has learned to go to sleep happily alone, but because he's exhausted and has despaired of getting help.'
Her claims reignite the ongoing debate with childcare guru and 'Contented Little Baby' author Gina Ford, who recommends that parents allow 'crying down' for up to ten minutes for babies who are so over-tired that they cannot settle themselves to sleep.