Flu cases in hospitals across England are seven times higher than last month, latest government figures shows.
Data from NHS England reveal there were 3,746 patients a day in hospital with flu last week, up from 520 a month ago.
And of those in hospital last week, 267 were in critical care beds, needing severe intervention to beat the virus.
Professor Sir Stephen Powis, NHS national medical director, said: "Sadly, these latest flu numbers show our fears of a 'twindemic' have been realised, with cases up seven-fold in just a month and the continued impact of COVID hitting staff hard, with related absences up almost 50% on the end of November."
The total number of NHS staff off sick is up a fifth on the end of November, from 52,556 to 63,296 a day - leading many hospitals and ambulance service to declare critical incidents.
The "critical incident" status means health bosses believe they cannot provide critical levels of care and patients could face harm.
Around a dozen NHS trusts and ambulance services have declared critical incidents in the last 10 days, with the health service facing “one of the toughest winters in its history”, officials have warned.
Dorset County, Portsmouth and Nottingham University Hospitals are the latest to make declarations on Thursday after warning they are being overwhelmed with patients while suffering staff shortages.
Industry officials said flu, Covid-19, industrial action and problems discharging patients are compounding the longer-term issues of poor investment, staff vacancies and a backlog of surgeries.
The NHS in Leeds said “some planned surgery will be cancelled” in order to prioritise urgent and emergency care, though it has not declared a critical incident.
Surrey and Sussex Healthcare Trust said on Wednesday it is seeing “record numbers of people attending A&E, calling NHS111, accessing GP services and calling 999” while more staff are off sick.
Medway Foundation NHS Trust in Kent and Wrightington, Wigan and Leigh NHS Foundation Trust in Lancashire also declared critical incidents last week.
Matthew Taylor, chief executive of the NHS Confederation, said: “The number of critical incidents we have seen declared over recent days are an accumulation of a number of difficult challenges meaning the NHS is facing one of the toughest winters in its history.
“High rates of flu, ongoing issues with delayed discharge and the disruptive impact of industrial action are compounding the longer-term issues of over 130,000 NHS vacancies, a decade-long lack of investment in capital and the elective backlog. This is bringing pressures to a head in many parts of the country.
“Whilst secondary care is where these challenges are perhaps the most visible, it’s also the case that primary, community and mental health care are under huge strain and many of the most vulnerable members of our communities rely on these services most.
“NHS leaders across the whole system are working hard to ensure patients get the best possible care and flow is optimised but the reality is the system is creaking under unprecedented pressure.”
Six ambulance services have also declared critical incidents since December 19, with North East Ambulance Service and East of England Ambulance Service declaring twice.
