Being a father, she says, 'is not a reward for good behaviour

 when Vicky Howard and her husband divorced last year, she was all for their 18-month-old daughter, Molly, spending every other weekend with him. But a year later, she would do anything to stop Molly spending nights at his flat.

"I'm not saying he shouldn't see her. But as for staying the night, I think it's harmful. She comes home out of sorts and I wonder what all the to-ing and fro-ing will do to her in the long term."

Vicky is not alone, with thousands of single mothers fearful about the effects on their young children of going for sleepovers at their dads.

"I'm really concerned, but people have told me it's politically incorrect and selfish to think like that and my ex's lawyers would surely laugh me out of court, so I generally keep my worries to myself," says one mother of a two-year-old, who wishes to remain anonymous.

One of Britain's leading parenting experts, Penelope Leach, says these mothers are right to be worried.

In fact, the psychologist, whose parenting books have sold millions, has made the controversial claim that children aged four and under should not be having sleepovers with their fathers at all when couples have separated.

In her new book, Family Breakdown, Leach claims that attempts by separated parents to 'share' young children is putting adult rights above those of children and that there is undisputed evidence that separating children from their mothers reduces brain development and creates a tendency towards unhealthy attachment issues.

"When people say that it's 'only fair' for a father and mother to share their five-year-old daughter on alternate weeks, they mean it is fair to the adults – who see her as a possession and her presence as their right – not that it is fair to the child," she says.

Leach, who has previously caused controversy by claiming that only mothers can care adequately for their children and that the paternal role is only of secondary importance, also took the opportunity to have a stab at the legal profession.

"When a lawyer bids for his client to have his baby or toddler to stay overnight each weekend, they are both ignoring clear evidence that such overnight separations from the mother are not only usually distressing, but also potentially damaging to the brain development and secure attachment of children under about four."

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Being a father, she says, 'is not a reward for good behaviour'.