Patients can expect a “winter of misery” with cold weather, unparalleled pressure on the NHS and potentially the worst flu season in two decades all expected over the coming weeks.

NHS Providers said the health service in England would be “sorely tested” during the colder months, when there was traditionally a significant rise in demand for services due to flu, norovirus, and respiratory conditions.

And figures released by Public Health England on Thursday demonstrate a “stark picture of what lies ahead”, according to Labour’s shadow health secretary Jonathan Ashworth, who cited statistics that show that more than 20,000 patients were stuck in the back of ambulances for 30 minutes or more during the first two weeks of winter.

NHS Providers, which represents health service acute, ambulance, community and mental health services, said that while winter planning in the health service in England was “extensive and more effective than ever before”, the NHS was “not where it would want to be” heading into winter.

It warned that this year’s flu strain was “potentially the worst we have seen in two decades”, with concerns that the UK should expect a similar flu outbreak to that seen in the Southern Hemisphere, where hospitals were forced to close their doors to new patients and people faced long waiting times.

Public Health England said there have been four confirmed flu-related deaths so far this winter.

Its latest figures show there were seven admissions of confirmed cases of influenza to intensive care or high dependency units in the week ending November 26.

While incentive payments for hospitals have seen 75% of staff vaccinated, more than half a million eligible healthcare workers are yet to be.

Niall Dickson, chief executive of the NHS Confederation, which represents organisations across the healthcare system, warned that hospitals are already nearly full and with a cold weather warning issued for this weekend the situation is likely to get worse.

There is a high probability of “severe cold weather, icy conditions and heavy snow” over the next few days in some areas, according to Public Health England, which could increase the health risk to vulnerable patients such as the very young, very old, or those with chronic diseases.

“These figures tell a very alarming story at this stage in winter, hospitals are nearly full, across England they are experiencing nearly 95% bed occupancy,” Mr Dickson said.

“This level of system pressure is quite simply unsafe and is putting patients at risk.

“There is a real human cost here.

“On two days in November there was only one paediatric intensive care unit bed available in the whole of London.

“Parents who have spent months preparing their children for urgent operations are turning up at hospital only to be told they have been cancelled.

“Add to that a 16% rise in norovirus cases compared with last year and more than 1,800 patients stuck in ambulances outside A&E departments in one week and we can clearly see a system which is struggling to cope.

“Extra money in the budget was welcome but £335 million was frankly too little too late, and it did nothing to ease the social care crisis.”

Mr Ashworth said: “After an inadequate Budget for the NHS, healthcare leaders warned of an imminent and unprecedented winter crisis.

“Today’s data reveals a stark picture of what lies ahead: a winter of misery for patients and unparalleled pressures on our NHS staff.

“Our new analysis of ambulance diverts reveals the dire impact Tory underfunding is already having on thousands of patients unable to promptly access A&E departments right across the country.

“Some Trusts are already completely full with no spare beds, an extremely worrying indicator of what is still to come particularly after NHS Improvement’s explicit warning that handover delays ‘should not occur’.”

Chris Hopson, chief executive of NHS Providers, said: “Winter always presents a big challenge to the NHS.

“Last year the pressures were intolerable. Services were stretched up to, and in some places beyond breaking point.

“This time preparations have never been more thorough. But we have to recognise we are not where we would want to be as we head into winter.

“The NHS is already under severe pressure, and while the additional funding in the recent Budget is welcome, it has come very late to be used to maximum effect.

“We cannot say with certainty how tough this winter will be, but the likelihood is that services will be sorely tested. We must hope the considerable efforts to curb the impact of flu are successful.”