The UK Health Security Agency is investigating 74 cases of hepatitis among children across the UK.
The health body is examining 49 cases in England, 13 in Scotland and 12 across both Wales and Northern Ireland, all of which have occurred since January.
The UKHSA said that it is investigating a "number" of possible causes of the cases.
One potential line of inquiry is whether or not a group of viruses called adenoviruses may be causing the illnesses.
Other possible explanations are also being investigated, including whether or not Covid-19 could have played a role in the spate of cases.
But officials stressed that there is "no link" to Covid-19 jabs as none of the children affected have received a Covid-19 vaccine.
Dr Meera Chand, director of clinical and emerging infections at the UKHSA, urged parents to be alert to the signs of hepatitis amid the rise in cases.
She said in a statement: "We are working swiftly with the NHS and public health colleagues in Scotland, Wales and Northern Ireland to investigate a wide range of possible factors which may be causing children to be admitted to hospital with liver inflammation known as hepatitis.
"One of the possible causes that we are investigating is that this is linked to adenovirus infection.
"However, we are thoroughly investigating other potential causes.
"Normal hygiene measures such as good handwashing, including supervising children, and respiratory hygiene, help to reduce the spread of many of the infections that we are investigating.
"We are also calling on parents and guardians, to be alert to the signs of hepatitis, including jaundice, and to contact a healthcare professional if they are concerned."
The UKHSA said that adenoviruses are a family of common viruses that usually cause a range of mild illnesses - including colds, vomiting and diarrhoea - and most people recover without complications.
While they do not typically cause hepatitis, it is a known rare complication of the virus.
Adenoviruses are commonly passed from person to person and by touching contaminated surfaces, as well as through the "respiratory route", the UKHSA said.
According to the NHS, hepatitis is the general term used to describe inflammation of the liver.
It's often seen in alcoholic patients with severe liver damage - but in the children's cases will be the result of a viral infection.
Hepatitis is an umbrella-term for a number of different types of liver inflammation.
Some types pass, while others can become chronic and cause scarring of the liver - or cirrhosis.
Chronic inflammation can also lead to a loss of liver function and liver cancer in some cases, according to the health service.
Acute hepatitis, which is short-term opposed to the lengthier timescale of chronic conditions, often has no noticeable symptoms.
But for the more severe cases, there are a number of key indicators parents can look out for.
Top 10 most common symptoms of hepatitis
- Dark urine
- Pale grey poo
- Itchy, irritated skin
- Jaundice - yellowing of eyes and skin
- Muscle and joint pain
- High temperature
- Feeling or being ill
- Unusual fatigue
- Loss of appetite
- Stomach pain
