Children aged under two who spend more than one and a half hours a day watching TV or online have weaker language skills and are more likely to misbehave at ages four and eight, a study has revealed.
In the pioneering research, academics tracked more than 6,000 children from aged two to eight to investigate the impact of breaching the limit for toddlers aged two to five of no more than one hour a day of screen time.
This is the maximum recommended by health chiefs, but most British children aged two spend more than two hours a day watching TV, playing on computers or using a smartphone. One in five already owns a smartphone by the age of two, according to the online regulator Ofcom.
The peer-reviewed research by University of Canterbury academics found that children who watched more than 90 minutes of "daily direct screen time" were, by the ages of 4.5 years and eight, below average for vocabulary, communication, writing, numeracy, and letter fluency.
They also had poorer social skills and higher levels of "peer problems." These were defined as being more likely to play alone, less liked by other children, less considerate and less likely to share their toys.
Children who had more than 2.5 hours screen time fared even worse. By contrast, children who watched less than the internationally recommended one hour a day screen time had better language skills, higher education levels, fewer behavioural problems and were more sociable than the average by the ages of four and eight.
The impact of breaching screen time limits was significant enough by the age of eight to mean that children were more likely to be in the bottom third of their class educationally. If they had not had so much screen time, they might only have been in the bottom half of the class.
"We found a linear relationship, so the more screen time they were exposed to, the worse they fared in terms of outcomes, and that was most evident at the highest levels of screen exposure," said Megan Gath, one of the researchers, said.
The report said the amount of time that children were increasingly spending online explained why teachers were seeing children coming into school at age five with lower language, education and social skills than in the past.
"Screen time during early childhood is predictive of the skills that children have on arrival to school, and the dramatic rise in screen use over recent years may partially explain why school readiness has been declining over recent years," said the researchers.
This was beyond and above the health risks from the greater time they were sedentary because of their screen time including higher rates of obesity, illness, and more frequent doctors visits, said the researchers.
They said that reducing the time children spent watching TV or online could have significant effects on the quality of children's education and health.
"Given the ubiquity of screen media in children's daily lives and the fact that usage is easily modifiable, the public health impacts are likely to be significant," said the report, published in Developmental Psychology.
Internationally agreed World Health Organisation (WHO) guidelines recommend no screen time for children under age two years and less than one hour per day for children between the ages of 2 and 5 years.
However, current research has consistently shown up to 80 per cent of two-year-olds and 95 per cent of three-year-olds internationally are engaging with digital screens above recommended levels.
The UK has introduced the Online Safety Act but this focuses on tackling what children view and experience online to protect them from harmful content rather than the time that they spend on the internet, playing online games or watching television.
Under pressure from Government and regulators, the tech companies have, however, introduced measures to remove or restrict technologies aimed at keeping children online for longer such as alerts, algorithms and endless scrolls.