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Another 20 children have been diagnosed with unexplained hepatitis in the UK, bringing the total number affected to 222, health officials say.
Of those cases, 158 live in England, 31 in Scotland, 17 in Wales and 16 in Northern Ireland.
Most are under five years old and had diarrhoea and nausea followed by jaundice - yellowing of the skin or eyes.
A surge in cases has also been detected in countries around the world.
The UK Health Security Agency (UKHSA) is investigating a strong link with a common virus, called adenovirus.
It has been detected in many samples taken from children, but health experts are also looking at other factors including the effect of previous infections, such as Covid, or two infections occurring at the same time.
A formal study of the causes and factors involved is under way, UKHSA said.
The UK first reported an unexpected rise in cases of severe liver inflammation, or acute hepatitis, in previously healthy children in mid-April.
It said there was no evidence of any link to coronavirus vaccines because the majority of infected children are too young to have received the jab.
More than 20 countries have since found children affected in a similar way.
Dr Renu Bindra, senior medical adviser and incident director at UKHSA, said: "We are working with other countries who are also seeing new cases to share information and learn more about these infections.
"The likelihood of children developing hepatitis remains extremely low.
"Maintaining normal hygiene measures, including making sure children regularly wash their hands properly, helps to reduce the spread of many common infections, including adenovirus.
"We continue to remind everyone to be alert to the signs of hepatitis - particularly jaundice, look for a yellow tinge in the whites of the eyes - and contact your doctor if you are concerned."
A small number of children over the age of 10 with hepatitis are also being investigated.
Some children in the UK have needed a liver transplant.
No children have died from the illness in the UK, but there have been deaths overseas.