The number of suspected UK norovirus cases is more than double the five-year average as a single strain of the disease appears to be driving up infections, public health figures show.
The UK Health Security Agency's (UKHSA) report also shows 105 suspected norovirus outbreaks have been reported this year - 27.4% higher than the five-season average of 82 suspected outbreaks.
Norovirus, sometimes referred to as the winter vomiting bug, is the most common infectious cause of vomiting and diarrhoea and spreads easily through contact with contaminated surfaces or someone else who has it.
UKHSA has said an increase in "norovirus activity" emerged earlier this year and reported cases are continuing to rise in all age groups - in particular in adults aged 65 and over.
It comes as there were 747 laboratory reports of norovirus between 18 November and 1 December this year - more than double the five-season average of 349 laboratory reports, the figures show.
UKHSA works out the five-season average by using data from the same time period during 2016, 2017, 2018, 2022 and 2023 - missing out years when coronavirus pandemic measures were in place.
The number of norovirus laboratory reports between the two-week period of 18 November and 1 December this year was also 33.2% higher than the fortnight before - when there were 561 laboratory reports.
These figures give an idea of the prevalence of the virus rather than an exact picture - as not all lab samples come back positive.
Out of the total number of laboratory reports this winter, 652 have been confirmed as norovirus.
The UKHSA says 63.2% of those were a subtype called GII.17.
The public health body says the figures show an increase in the "proportion of samples characterised as GII.17, with a similar trend noted in the United States and other European countries".
The UKHSA has said: "It is likely that multiple factors contributed to the observed increase in laboratory reports, such as ongoing changes to the epidemiology following the COVID-19 pandemic or changes in testing and reporting to national surveillance.
"Further work to understand the drivers of the increased reporting is under way."
Meanwhile, the public health body said 85 of the 105 suspected outbreaks reported to the Hospital Norovirus Outbreak Reporting System (HNORS) this year were confirmed as being norovirus.
This compares to a five-season average of 70 cases.
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'Quad-demic' warning
The UKHSA has published the norovirus figures as four illnesses have been circulating in the UK this winter in what has been dubbed the "quad-demic".
NHS leaders issued a warning that cases of flu, norovirus and respiratory syncytial virus (RSV) are increasing, with a rise in COVID-19 also expected.
Data released by the health body showed an average of 1,861 patients with flu were in hospital every day in the first week of December - a 70% increase from the previous week when there were 1,099 patients.
The number was also 3.5 times higher than the same time last year.