The number of known cases of sudden onset hepatitis among children under 10 has risen to 111 and the number of those who have needed a liver transplant has increased to 10, the UK Health Security Agency has reported.
It said there have been no deaths in the UK.
The World Health Organisation (WHO) reported one death of a child with hepatitis this weekend but did not say where it had occurred.
Most of the cases of liver inflammation were in children under five, though a small number of cases in children over 11 are also being examined.
Of the confirmed cases, 81 live in England, 14 are in Scotland, 11 are in Wales and five are in Northern Ireland.
The cases are predominantly in children under five who showed initial symptoms of diarrhoea and nausea followed by jaundice.
Health agencies are urgently investigating the cases and say they do not know what is driving this surge.
A number of hypotheses are being considered as to why there have been cases of a more severe infection of adenovirus, leading to the rise in hepatitis.
They include children being more susceptible to the virus as a result of a lack of exposure to it during the pandemic, a prior infection to COVID or another illness, a co-infection with COVID or another illness and toxin, drug or environmental exposure.
Other factors include a novel variant of adenovirus.
Officials said the cases have not been caused by the usual viruses that cause hepatitis A – E and data gathered has suggested that the rise in severe cases of hepatitis may be linked to a group of viruses called adenoviruses.
Of 53 cases tested, 40 (75%) showed that adenovirus was the most common pathogen detected. But these adenoviruses usually cause mild flu-like symptoms and not acute liver failure.
A total of 16% of cases, meanwhile, were positive for COVID-19, which was not unexpected due to the high rates of the virus from January to April this year.
The agency said there is no link to the coronavirus vaccine as none of the currently confirmed cases in under 10s in the UK are known to have been vaccinated.
Routine NHS and laboratory data shows that common viruses circulating in children are currently higher than in previous years and there is a marked increase of adenovirus, particularly in the one to four age group, it added.
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Dr Meera Chand, director of clinical and emerging infections at UKHSA, said: “Information gathered through our investigations increasingly suggests that this rise in sudden onset hepatitis in children is linked to adenovirus infection. However, we are thoroughly investigating other potential causes.
“Parents and guardians should be alert to the signs of hepatitis, including jaundice, and to contact a healthcare professional if they are concerned. Normal hygiene measures such as thorough hand washing, including supervising children, and good thorough respiratory hygiene help to reduce the spread of many common infections, including adenovirus.
“Children experiencing symptoms of a gastrointestinal infection including vomiting and diarrhoea should stay at home and not return to school or nursery until 48 hours after the symptoms have stopped.”
Similar cases have been found across Europe, Israel and the United States.