- Video report by Social Affairs Editor Penny Marshall
The prime minister has apologised for delays to operations and hospital admission as the NHS in England struggles to cope amid winter pressures.
Hospitals have been struggling with mounting demands on their services and tens of thousands of operations could be delayed for at least a month.
During a visit to Frimley Park Hospital in Surrey, Theresa May said she recognised it was "difficult" for anybody who has had their operation postponed.
Mrs May added that she hoped procedures could be rescheduled "as soon as possible".
"I know it's difficult, I know it's frustrating, I know it's disappointing for people, and I apologise."
The prime minister also thanked NHS staff across the country for their "fantastic work".
- Video report by ITV News Correspondent Dan Rivers
It comes as new figures revealed delays in ambulances getting patients to A&E departments in England have peaked at their highest level so far this winter.
The figure was up from 11,900 the previous week, and included 4,700 patients delayed for more than an hour.
The number of people waiting more than an hour to be handed over to A&E staff during Christmas week nearly doubled on the week before.
- Penny Marshall explains that this year's winter pressure feels like a "significant difference" on previous years, adding that she believes the NHS has reached a "tipping point".
The Department of Health says ambulance crews should be able to hand patients over to A&E staff within 15 minutes of arrival at hospital.
Failures to do so increases the risk to patients due to delays in diagnosis and treatment, as well as the chance their condition could deteriorate.
- NHS 111 non-emergency calls reached record high
NHS England's weekly operational update also showed non-emergency calls to the health service's 111 hotline again reached a record high in the week ending December 31.
Calls to the service were up 21% on the previous week to 480,400 - the most received in a single week since the hotline was created.
Bed occupancy rates climbed as high as 93.5% on New Year's Eve, up from 86.7% on Christmas Day, according to the data.
The levels reached an average of 91.7% across the week.
In week before hospitals had reported bed occupancy levels of 90.9% - above the recommended safe limit of 85%.
Niall Dickson, chief executive of the NHS Confederation which represents organisations across the healthcare system, warned that figures do not reflect the scale of the problem facing the health service.
He said: "Staff are working at full capacity to deliver the right care, but the pressures are becoming intolerable.
"The stats also mask the pressures which can be seen across all parts of the system - in social care, community and mental health services, as well as at the hospital front door and in our ambulance services."
"If the health service cannot cope at its front door, what lies behind it will also be struggling," he added.
An NHS England spokesperson said: “Hospitals, GPs, ambulances and other frontline NHS services have been extremely busy between Christmas and New Year, reporting higher levels of respiratory illness and some indications of increasing patient illness severity and flu."
"These increased pressures were mirrored in the NHS 111 service."
"In the light of these pressures the medical and nursing-led National Emergency Pressures Panel has now enacted, for a time-limited period, the NHS' Winter Pressures Protocol to free-up further staff and beds for patients needing urgent and emergency care."