The NHS has disputed claims from the Red Cross that there is a "humanitarian crisis" in its hospitals in England.
One of NHS England's specialist medical directors said he thought the service was not "at that point" of crisis, but admitted demand was higher than ever.
The Red Cross helps some hospitals with patient transport and provides care for patients who have returned home.
Labour's Jeremy Corbyn urged Theresa May to tell MPs how she would fix the "national scandal" of the NHS.
Mr Corbyn said: "The fact is, this government have repeatedly failed to put the necessary resources into our health service, while they have cut social care and wasted billions on a top-down reorganisation to accelerate privatisation."
The Labour leader said he was "demanding" that Theresa May appeared at the Commons on Monday to explain "how she plans to fix her failure on the NHS".
It comes as a third of hospital trusts in England warned they needed action to cope with patient numbers last month.
Figures show that 42 A&E departments ordered ambulances to divert to other hospitals last week - double the number during the same period in 2015.
Diversions can only happen when a department is under significant pressure, such as lacking the capacity to take more patients or having queues of ambulances outside for significantly prolonged periods, and when all existing plans to deal with a surge in patients have been unsuccessful.
The Royal College of Emergency Medicine said staff were under intense pressure, while the Society for Acute Medicine warned this month could be the worst January the NHS had ever faced.
Its president, Dr Mark Holland, told BBC Breakfast that the term "humanitarian crisis" was strong, but "not a million miles away from the truth".
He said: "We have been predicting that we would face a winter from hell. I think that time has arrived."
Professor Keith Willett, medical director for acute care at NHS England, said he did not think the service was at the level of a humanitarian crisis, but admitted demand was at its highest level ever and staff were under "a level of pressure we haven't seen before".
Speaking to BBC News, he would not answer whether the government was taking reforms to social care of patients returned from hospital seriously, but said that was where future investment needed to go.
"We have to transform and modernise the way the NHS works and we do have to move much more care into the community where it is much more appropriate, particularly for the elders in our society," he said.
"But there is no doubt… that if there is more money to be had, then it should be directed at social care in the community. That will help the NHS more than anything else at the moment."
Worcestershire Acute Hospitals NHS Trust said on Friday that it was investigating two deaths at Worcestershire Royal Hospital's A&E department in the last week.
The trust said patient confidentiality prevented it from discussing the deaths, but added it had "robust plans" to maintain patient safety and emergency care. A patients' watchdog has called for an investigation.
John Freeman said his wife Pauline, who is recovering from a stroke, spent 38 hours on a trolley at the same hospital because of overcrowding.
"My wife was stuck on a trolley right next to the fire doors in a corridor and she couldn't get any sleep because of all the trolleys banging into the fire door going in and out," he told BBC News.
"There was probably in excess of 20 trolleys all stacked up. This is going back to the dark ages almost."
Trusts around the country are taking to social media to urge patients to stay away from A&E, unless it is an emergency or a life-threatening illness.
The British Red Cross provided support to staff at the East Midlands Ambulance Service across Nottingham, Leicester, Lincoln, Kettering and Northampton on 1 January.
It also boosted existing services offering support at home to help alleviate pressure on hospitals.
Chief executive Mike Adamson said: "The British Red Cross is on the front line, responding to the humanitarian crisis in our hospital and ambulance services across the country.
"We have been called in to support the NHS and help get people home from hospital and free up much needed beds.
"We've seen people sent home without clothes; some suffer falls and are not found for days, while others are not washed because there is no carer there to help them."
He said that if people did not get the care they needed "they will simply end up returning to A&E, and the cycle begins again".
Speaking to the BBC, Mr Corbyn said it was "unprecedented" for the government to be criticised by the Red Cross, which he said was "essentially a volunteer organisation".
He said there was a "crisis" both in social care and hospital funding which needed to be dealt with "urgently".
"It needs government intervention now," he said.
"We have health care as a human right in this country - that's what the NHS is for. The NHS needs the money now in order to care for everybody."
What help does the Red Cross give to hospitals?
The Red Cross offers a 'support at home' service to hospitals that need to improve the flow of people in and out of hospital.
Volunteers visit trusts to see what social care needs patients have when they are discharged. They then visit them at home and help them with tasks including collecting prescriptions, doing shopping or simply offering company.
It says the number of patients its volunteers see has gone up by 10% year-on-year and the range of tasks is increasing, such as making sure people eat, helping them to get dressed or assisting them in going to the toilet.
At one trust, the organisation has been working alongside hospital matrons to arrange transport for people to take them home. It uses its fleet of Land Rovers and also provides back-up for the ambulance service.
The Red Cross works in more than 100 hospitals on an ongoing basis.
Services rendered to the NHS are paid for on contracts of up to five years.
The Red Cross says the money goes towards the cost of the services it provides, plus a small amount towards its charitable aims.
SOURCE: Red Cross
Speaking on BBC Radio 4's Today programme, Mr Adamson said his volunteers were facing an "increasingly complex and chaotic situation", where they had to do much more to help people get back on their feet at home.
"Money is definitely an issue because of all those people who used to get social care support who no longer do," he said.
"There does need to be more funding for social care.
"The NHS is doing a fantastic job but needs to be more open to working with voluntary sector partners who can make a real difference to improve the flow of patients through the system and address some of these issues.
"Critically, [they can] improve the sense of wellbeing and reduce the sense of crisis with those individuals so they can live independently for longer."
Dr Taj Hassan, president of the Royal College of Emergency Medicine, said every hospital in Essex has been on black alert and emergency departments are "working at and beyond their capabilities".
He said: "The emergency care system is on its knees, despite the huge efforts of staff who are struggling to cope with the intense demands being put upon them. This cannot be allowed to continue.
"The scale of the crisis affecting emergency care systems has reached new heights, as we predicted, mainly due to a lack of investment in both social and acute health care beds, as well as emergency department staffing."
A spokesman for NHS England said plans remained in place to deal with additional demands during the winter period and asked the public to "play their part" by using local pharmacies and NHS 111 for medical advice.
He added that beds were not as full as this time last year.