Children under the age of 12 have been banned from heading footballs during training.
Players will be allowed to use their heads during matches, but headers will be banned in practice or training sessions.
The new guidelines apply to children in England, Scotland and Northern Ireland, but not Wales.
The new rules have been brought in following research that shows a possible link between football and the brain condition, dementia.
In the USA, children under 10 are already banned from heading footballs and there are also restrictions on players aged 11-13 doing headers during football training.
Last year, former England player Ryan Mason called for the ban.
This is because he was forced to retire after fracturing his skull during a challenge in a Premier League game.
He now coaches Tottenham Hotspur's under-18 team and suggested that children should use sponge balls to learn the technique rather than heading harder footballs.
A study released in October 2019 reported the first possible links between former footballers and brain disease.
The report suggested players could be three and a half times more likely to die of dementia.
Although there remains no firm evidence linking heading the ball to dementia, the Scottish FA's doctor John MacLean says limiting head contact for younger players is common sense.
"We need to take some sensible, pragmatic steps at the moment and that's largely going to be about trying to reduce that overall burden, the overall [number of] times that young players head - and heading in training is much more common than in matches," he said.
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