An outbreak of "superflu" which is most common among children is the UK's biggest health challenge since the pandemic, Health Secretary Wes Streeting has warned.
Some schools in the UK have brought back Covid-era measures to prevent the spread of the illness, by either reducing hours or shutting temporarily.
Health authorities have also suggested that people consider wearing masks in public.
National Health Service figures showed flu cases at a record level for the time of year after jumping 55 per cent in a week to an average of 2,660 patients in hospital each day last week.
Children aged five-14 were most likely to test positive for flu, followed by 15-24 year olds, then those aged 25-44. Those aged over 45 were least likely to have the flu.
However, hospital admission rates for flu in England overall are highest among over 75s and under fives.
The NHS is expected to come under further pressure as schools break up and families gather for the Christmas break, with many returning from abroad. With the UAE school term ending on December 5, many families have already arrived in Britain from the Emirates.
Mr Streeting said cases could triple and there was “a tidal wave of flu tearing through our hospitals”.
This year's flu season involves the H3N2 subtype strain of influenza A, which was prevalent during Australia and Japan’s winter, seen as a reliable indicator of what the rest of the world faces as the virus moves north.
Spain has also recorded an extremely high number of cases, while the Irish government has expressed concern at the low take-up vaccines this year. Cases have surged in Central and Eastern Europe, notably in the Czech Republic, Poland, Hungary and Germany.
Hans Kluge, Europe director of the World Health Organisation said flu season started earlier in several countries across the continent.
"Children are driving transmission and adults 65+ are most likely to be hospitalised," he said. "Getting vaccinated is the best way to stay protected."
The UAE introduced its annual flu campaign in late September, calling on young children, the elderly and vulnerable groups to be vaccinated.
Prof Meghana Pandit, NHS medical director, warned the “unprecedented wave of superflu is leaving the NHS facing a worst-case scenario for this time of year”, with no peak in sight.
Dr Vicky Price, president of the Society for Acute Medicine, said the “flunami” was hitting the NHS but this was “a sadly familiar picture of a system under relentless strain”.
Mr Streeting has warned resident doctors who are due to strike at Christmas that their action could be “the Jenga piece” that forces the NHS to collapse.
Writing in The Times on Friday, he said a huge jump in flu cases has left the NHS facing a “challenge unlike any it has seen since the pandemic” and urged resident doctors to accept the government’s offer and end their action.
He said: “The whole NHS team is working around the clock to keep the show on the road but it’s an incredibly precarious situation. Christmas strikes could be the Jenga piece that collapses the tower.”
Mr Streeting described the scenes in hospital as “inexcusable”. He said British Medical Association (BMA) leadership calling off planned strikes would have “given the NHS certainty this week, when it is firefighting the flu epidemic”.
Weekly flu numbers in England peaked at 5,408 last winter and reached 5,441 over the winter of 2022/23, the highest since the pandemic.
Some hospitals have asked staff, patients and visitors to wear face masks to cut the spread of flu, while others have gone in and out of critical incident status due to the high number of people attending A&E.
Separately, an average of 354 hospital beds were filled each day last week by patients with diarrhoea and vomiting or norovirus-like symptoms, up 35 per cent from 263 the previous week.
This is lower than at this point last year but NHS England said winter viruses were beginning to “engulf hospitals”.
Prof Pandit said: “This unprecedented wave of super flu is leaving the NHS facing a worst-case scenario for this time of year.
“The numbers of patients in hospital with flu is extremely high for this time of year. Even worse, it continues to rise – and the peak is not in sight yet.
“With record-12-hour waits continuing to mount – 50,648 last month, up 10.6 per cent on last year – we are witnessing the appalling reality of corridor care on an unprecedented scale.
“While the ongoing ‘flunami’ is being invoked as the primary cause of the crisis, this is a convenient scapegoat when the reality is that the situation is a direct result of a long-standing and predictable breakdown in NHS capacity and workforce shortages."
Elsewhere, pharmacy organisations insisted they have good stocks of the flu vaccine after reports some patients were struggling to get one.
Dr Leyla Hannbeck, chief executive of the Independent Pharmacies Association, said: “Independent pharmacies across the country are reporting a big surge in inquiries about flu vaccinations and treatment. In some areas the vaccine stock is depleting, but most pharmacies have sufficient stock.”
The predominant strain of flu circulating this winter, the H3N2 strain, means there is “less natural immunity in the community”, the UK Health Security Agency said.
“H3N2 flu strains typically affect older adults more severely than H1N1 strains, leading to more hospitalisations and deaths, creating further pressure on our NHS this winter.”
A total of 71.7 per cent of people in England aged 65 or older are estimated to have received this year’s flu vaccination by December 7, according to UKHSA data.
