Cases of norovirus are up by almost 40% in England as hundreds of outbreaks of the winter vomiting virus have spread through schools and care homes in the past month.
Public Health England said on Thursday norovirus cases were 37% higher than normal for this time of year.
Cases dropped to extremely low figures during lockdown but have risen sharply after restrictions came to an end.
In July, England's chief medical officer, Professor Chris Whitty, said winter "could be tricky" due to the double whammy of higher numbers of seasonal viruses like norovirus as well as COVID-19.
Senior NHS chiefs have also warned the health service is heading for one of the most difficult winters in its history.
The report covered the period between 30 August and 26 September.
In the previous report covering August, norovirus cases were 55% higher than during a normal year.
The majority of outbreaks in the latest report were from educational settings (61%), particularly nurseries.
The authors said that in the five previous normal seasons (before the pandemic) the most common place for a norovirus outbreak was in a care home.
The report said: "It is possible that further unusual or out of season increases in norovirus activity could be seen in the coming months."
Watch: COVID-19: UK has little 'headroom' for coronavirus cases to surge before NHS 'heavily stressed', professor warns
Medical experts have warned the UK does not have much "headroom" for rising COVID-19 cases before the NHS comes under intense pressure.
Professor Neil Ferguson, a member of the Scientific Advisory Group for Emergencies (Sage), said when discussing comparisons with European countries: “We are much closer to the limit of what the NHS can cope with.
"We will come on to plan B, I think that is what is exercising Whitehall and policymakers, is that limited headroom.”
He said there was a high level of unpredictability in the modelling but added: “We could see continued flat incidence, even slow decline if we get boosters out quickly.
“So it’s not guaranteed we will see a large winter surge by any means, but we can’t afford, at the current time, to have too much of a winter surge before really the NHS is very heavily stressed.”
Asked about what other countries were doing, and whether the UK wants to keep case rates down, Prof Ferguson said: “The Government clearly has said, it’s not really science here, it’s a political judgment, they want to live with Covid.
“Their prime criteria for acting is additional pressure on the NHS.”
What is norovirus?
Norovirus is highly infectious and causes vomiting and diarrhoea but usually passes in a couple of days.
It is sometimes called the "winter vomiting bug."
How do you catch norovirus?
According to the NHS, you can catch norovirus easily from close contact with an infected person, touching surfaces an infected person has touched or eating food prepared by an infected person.