Keir Starmer today vowed to 'fight' tech firms over 'addictive' social media in efforts to keep children away from mobile phones, tablets and TV screens.
The Prime Minister said Labour needed to be 'clear whose side we're on' in a battle with platforms to limit the amount of time youngsters spend online.
It came as the Government issued guidance that advises parents of younger children to limit screen time for two to five-year-olds to an hour a day, and less if possible.
For children under two, screen time should be avoided unless for shared activities that encourage children to interact with others, parents have also been advised.
On a visit to a primary school in south London, Sir Keir said parents had been left searching for what is appropriate screen time for their children without any help.
'When there's a lot going on, when children are having a tantrum, trying to find something to distract them is an obvious thing,' the PM said on Friday.
'And I don't think parents and carers have had any guidance at all yet about what would be appropriate, what might be best.'
But some of the bored-looking youngsters Sir Keir met on his visit already appeared to have switched off; one was photographed gazing the other way while he spoke - and another was seen taking a nap.
The PM said Labour needed to be 'clear whose side we're on' in a battle with platforms to limit the amount of time youngsters spend online.
The Government is also currently consulting on how to protect children under the age of 16 online, which could include an Australia-style social media ban or measures like time limits and curfews.
Sir Keir added: 'Some of this will require a fight. If we're going to do more to protect children, we're going to have to fight some of the platforms that are putting the material up there because they're putting this addictive stuff up there for a reason.
'They want more children to spend more time online and we've got to fight them and be clear whose side we're on here.'
The new guidance on screen time for the youngest children advises families to avoid fast-paced social media-style videos and toys or tools that use artificial intelligence (AI), and keep bedtimes and mealtimes screen-free.
The guidance was developed by a panel led by children's commissioner Rachel de Souza and children's health expert Russell Viner.
Prof. Viner, a paediatrician and professor at University College London, said he would next like to see the Government issue more specific advice to parents on what is actually good content for their children to see.
He said: 'What we have suggested to the Government is better guidance from educationalists on what's good content and actually what isn't because there's a lot of educational content out there making all sorts of claims that can be confusing for parents.'
Prof. Viner said long periods of watching screens without interactive engagement 'crowd out' important things for young children's development like sleep and physical activity.
It also seems to have an impact on children's language development, he added.
Around 98 per cent of children are watching screens on a daily basis by the age of two, the Government has previously said, and those with the highest screen time see an impact on their language.
Appearing on ITV's Good Morning Britain, education minister Josh MacAlister said there has been 'a complete rewiring of childhood' over the last decade due to social media and screen time.
'For parents of really small children, we're trying to help create some new social norms,' he said.
'There's no judgment. It's much easier to say these things than it is to do them, I appreciate.'
A jury ruled on Wednesday in Los Angeles that Google, owner of YouTube, and Meta, which runs Facebook, Instagram and WhatsApp, built platforms to hook young users without regard for their wellbeing.
Sir Keir told reporters on Thursday he was very keen to tackle addictive features on social media.
Prof.Viner said evidence from Australia and from a trial on different restrictions will help the UK to make decisions on how to protect young people.
'We absolutely need to do something to limit the exposure of adolescents to harmful content, as that's the real issue,' he said.
'I absolutely support thinking about how we get social media companies to design things that are pro-development by design.'