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Mums and dads who spend more than an hour a day scrolling on their phones could be making their child less intelligent, a study has suggested.

Parental screen time had a greater effect on the speech of children aged five to seven speech and language skills than the frequency of device use by the children themselves.

Five-year-olds who owned tablets or phones were also found to have poorer speech than those who didn't.

However, the differences in ability linked to children's screen time vanished by the time they turned seven.

The Norwegian researchers warned that smartphones and tablets are attention disrupters' – and using them often may make parents less communicative with their children.

This limits opportunities to develop vocabulary at a crucial stage of language development.

Around 90 per cent of brain growth happens before the age of five, studies show.

Earlier this year the Government warned that excessive TV time could damage a toddler's ability to speak, and advised parents to limit screen time in children aged two to five to no more than one hour daily.

Just an hour of scrolling on your phone can wreck a child's language skills, research suggests

The research – published in the journal JAMA Pediatrics – followed 747 Norwegian children for three years, from the age of five until their eighth birthdays.

At the start of the study, parents were asked how many hours, on average, their children spent looking at screens, and if they had their own tablet or phone.

Mums and dads were also questioned on their own social media use and instructed to submit their screen time as recorded by their phone.

Children completed language tests once a year at ages five, six and seven which assessed how many words they could understand and how well they understood grammar.

Rather than comparing one-off scores, the team tracked how children's language ability changed over time, allowing them to distinguish those who started with weaker skills from those whose development slowed over time.

After adjusting for potentially confounding factors, including household income and parents' education levels, the researchers, from the University of Oslo, found that parental phone use was linked to slower vocabulary development.

Specifically, vocabulary development slowed with every hour a parent spent on their phone per day.

Children whose parents spent four hours a day glued to their phones scored around 2.5 points less on language tests, compared to those whose parents used their phones for around one hour a day.

Youngsters with their own device – 43 per cent of the group – also had lower language scores at the start of the study, but not at the end.

Dr Junyi Yang, an expert in special needs education and the study's first author, said: 'Smartphone use is often attentionally disruptive. Studies of parent-child interaction show that such disruption can affect children's word learning and language development.

'Greater overall screen use reflects a reduced opportunity for responsive interactions in the home.'

On average, researchers found the more time a parent spent on their phone, the higher a child's screen time tended to be.

The researchers concluded: 'Higher overall screen engagement within the family may replace high-quality interactions, such as shared reading and storytelling activities that promote word learning.'

It follows a study published in January that found that children aged two with the highest screen use – around five hours a day – could say significantly fewer words than those who spent less than an hour looking at screens.

Using devices is now near-universal in early childhood, the researchers found, with 98 per cent of two-year-olds now watching screens daily.

The World Health Organisation recommends limiting screen time to a maximum of one hour a day for children aged two to four.