This means that while a 20-year-old man passes on about 25 mutations through his sperm, in an average 40-year-old this will rise to about 65.

Kari Stefansson, senior author of the study by DECODE Genetics, an Icelandic company, said: "All areas of the human genome were a mutation once upon a time, so all human variety is down to a mutation.

"But one interesting aspect of this work is it shows us that the classic focus on the age of the mother and the health of the child is not sufficient.

"The increasing age of the father has a much bigger impact on a child's health in a general way. Women are off the hook and we men are on it."

Dr Allan Pacey, Chairman of the British Fertility Society, said: "It is a surprise to find that men transmit a higher number of mutations to their children than do women.

"Whilst not wanting to scare the children of older fathers, information like this is important to understand and should remind us that nature designed us to have our children at a young age and if at all possible men and women should not delay parenthood if they are in a position not to.

But Prof Darren Griffin, a geneticist at Kent University, said: "There are three billion of letters in the DNA code of humans and the numbers of mutations detected in this study are in the dozens.

"The observed approximate doubling of mutation rate between the ages of 20 and 40 (when most fathers are actively reproducing) is certainly clinically noteworthy but not realistically likely to deter more mature fathers from having children."

In an accompanying opinion piece in Nature, Prof Alexey Kondrashov of Michigan University suggested that younger men ought to bank their sperm to protect their future children from disease.

But Prof Christopher Barrett, an expert in reproductive medicine at Dundee University, said: "Whilst sperm banking is technically feasible, more data would be required to recommend this policy as routine."

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Older fathers pass on more genetic faults

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