The proportion of infants arriving at school not knowing whether they are right or left-handed has trebled in the past decade, researchers say. The situation has been made worse by excessive parental fears, driven by cot death, about letting them lie or crawl on their front.

Children of four and five are struggling to make advances in writing because of their stunted dexterity, made worse by shortening attention spans.

The trend has raised concerns that children are developing more slowly than in past years, leading to “indelible” behavioural problems in adolescence.

Madeleine Portwood, a senior educational psychologist at Durham county council, said that from her observations of hundreds of children, the proportion of those who started school not knowing whether they were more comfortable holding a pencil in their left or right hands had grown from 10% a decade ago to 25%-30%.

“It’s important if you start formal education at 4½ and you are expected to hold an implement to write, that you know which hand to hold it in,” she said.

Portwood believes an important factor in the change is that some parents interpret advice that children should sleep on their backs to avoid cot death to mean that they should never be allowed on their fronts, even when awake and on the floor.

This means infants are less likely to crawl on their hands and knees and develop left-right coordination between arms and legs as they learn to stand and walk.

Portwood, who presented her findings at an independent schools conference last week, said: “More and more children are not going through the crawling stage. They shuffle along on their bottoms and find a chair, a table or curtains and use their arms to pull up to a standing position.

“The most important thing parents can do is ensure that when they are being observed during the day, they are given a chance to be on their front.” Previous research by Portwood has found that 57% of three-year-olds are unable to carry out tasks expected at their age. She cited children’s inactive lifestyles as “a major contributory factor”.

Other experts have also raised concerns about children’s development. “Brain development is at its most rapid between the age of zero and three,” said Aric Sigman, a psychologist and a fellow of the Royal Society of Medicine. He pointed to research showing that for every hour a day a three-year-old watches television, there is a 9% rise in attention problems.

Sigman has described television as “the greatest unacknowledged public health issue of our time”. He also believes video games have led to children spending less time working with their hands and failing to grasp concepts such as weight, volume and measurement.

“By using your hands, you can actually become more civilised,” said Sigman. “These are problems likely to persist in life, they are rather indelible.”

The problem was highlighted at the Conservative party conference when a restaurateur told a session addressed by David Willetts, the shadow skills secretary, that she was unable to find British employees under 25 who had the dexterity to peel a potato.

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