Hospitals have put extra beds in place this year to help deal with the pressure from flu and other winter viruses, including Covid and norovirus (the winter vomiting bug).
NHS chiefs say they have also added extra support for people who frequently need emergency services and more care is being delivered outside hospitals.
NHS Confederation chief executive Matthew Taylor said: “The NHS has done all it can in advance to mitigate risks to patients this winter, but we should be under no illusions that the service is in a position of national vulnerability as the intense pressures we are now seeing start to grip local services.”
Figures for last week show some 12,200 patients were left waiting in ambulances for more than an hour before they could be cared for in hospital.
Dr Adrian Boyle, head of the Royal College of Emergency Medicine, told Radio 4’s Today programme that it was a very difficult time for both patients and staff.
“What we are seeing is ambulances are frequently stuck outside emergency departments and our emergency departments are full.”
He said there was “an enormous amount of avoidable harm – excess deaths that shouldn’t be happening”.
“Flu is the straw that is breaking the camel’s back because we have this chronic lack of beds within our hospitals, and we don’t use those beds properly because we haven’t reformed social care,” he added.
His criticism comes on the day the health and social care secretary set out plans for reforming adult social care in England, although they are unlikely to be delivered before 2028 at the earliest.
A number of hospitals in different regions in England have restricted visits and asked patients and visitors to wear face masks to prevent the spread of flu.
In Scotland, there has also been a large rise in the number of people in hospital with flu. Earlier this week, First Minister John Swinney said the NHS was under “very significant pressures”.
A Welsh government spokesperson said there were continued “high levels of demand” on urgent and emergency care services this winter and an increase in flu cases had added further pressure during the Christmas period.