Experts may have discovered the cause of a recent increase in mysterious liver problems in young children around the world.

Research by teams in London and Glasgow suggests that two common viruses have made a comeback following worldwide lockdowns, and these have triggered the hepatitis cases.

It is believed more than 1,000 children in 35 countries have been affected, with some needing liver transplants.

According to the researchers, because of Covid restrictions, infants have been exposed later and therefore missed out on early immunity to adenovirus, which normally causes colds and stomach upsets, and adeno-associated virus two which normally causes no illness and requires a coinfecting "helper" virus - such as adenovirus - to replicate

Professor Judith Breuer, an expert in virology, at University College London and Great Ormond Street Hospital, and part of the research team said: "During the lockdown period when children were not mixing, they were not transmitting viruses to each other.

"They were not building up immunity to the common infections they would normally encounter."When the restrictions were lifted, children began to mix, viruses began to circulate freely - and they suddenly were exposed with this lack of prior immunity to a whole battery of new infections."

According to Professor Emma Thomson, who led the University of Glasgow research, there are still many unanswered questions. "Larger studies are urgently needed to investigate the role of AAV2 in paediatric hepatitis cases.

"We also need to understand more about seasonal circulation of AAV2, a virus that is not routinely monitored - it may be that a peak of adenovirus infection has coincided with a peak in AAV2 exposure, leading to an unusual manifestation of hepatitis in susceptible young children."