Doctors are worried a combat sport called slap fighting, watched by millions online and gaining in popularity, is causing serious brain damage.

Competitors face off and take turns to deliver bare, full-force, open-handed strikes to the cheek.

To assess the possible harm, medics screened videos of tournaments and have now written a warning letter in a leading journal, JAMA Surgery, external, about their findings.

Unlike boxing, no head gear is allowed and defenders cannot duck to avoid blows. Even flinching is banned.

Opponents are scored based on the amount of damage they inflict and how well they cope with getting slapped themselves.

The sport is popular in the US, but other countries, including the UK, are in on the action.

Next month, Liverpool will hold what is being billed as the first-ever British Heavyweight Slap Fight Competition.

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It is not the first warning public about the sport though.

The alarm was raised in 2021 after Polish slap fighter Artur Walczak suffered a brain bleed during a match in which he was knocked out and lost consciousness.

Despite hospital treatment, he died weeks later of multiple organ failure linked to his head injury.