Health chiefs in Hampshire have revealed they have experienced an "unprecedented" influx in patients displaying flu-like symptoms.
More than half admitted through Southampton General Hospital's emergency assessment unit have displayed influenza-related illnesses.
And about 10 patients a day have been referred for respiratory examination.
The trust warned it had put a "major strain" on resources, with figures likely to increase throughout January.
It comes as the government relaunched its national flu prevention campaign in an attempt to quell the rising number of people being diagnosed with the potentially-killer illness.
Nationally 39 people in the UK have now died with flu-like illnesses this winter, figures have shown.
Dr Ben Marshall, a respiratory specialist and flu expert, said the hospital had seen a "week-on-week" increase in patients admitted with influenza-like illness in Southampton.
"Currently we have over 30 adults alone now in hospital receiving treatment, that is unprecedented at this time," he said.
"There is currently lots of seasonal flu around in the community, but what is concerning is that we are taking in many patients under 65 with the H1N1 strain who are also contracting pneumonia, creating further complications."
"While we did expect an increase in flu-related hospital admissions this year, the numbers have been far higher and there is no sign of that letting up any time soon."
Dr Marshall said many people will be able to self-treat at home and advised people, particularly those in at-risk groups such as the elderly or those with chronic illness, as well as pregnant women, to contact their GP and get vaccinated.
"This situation is likely to continue throughout January as children return to school and adults to work after the festive break, so it would be prudent for those in at-risk groups to have their seasonal flu vaccination as soon as possible."
Meanwhile, three wards at Southampton General Hospital remain temporarily closed to new admissions due to outbreaks of diarrhoea and vomiting.