Video report by ITV's National Editor Allegra Stratton
Patients are "dying prematurely" in the corridors of A&E units, the heads of more than 60 emergency departments have warned Theresa May.
The "current level of safety compromise is at times intolerable, despite the best efforts of staff," a letter signed by consultants in charge of emergency departments in 68 acute hospitals across England and Wales said.
Acknowledging the best efforts of staff, trusts and clinical commissioning groups, it adds: "The facts remain, however, that the NHS is severely and chronically underfunded.
"Thousands of patients are waiting in ambulances for hours as the hospitals lack adequate space.
"Some of our own personal experiences range from over 120 patients a day managed in corridors, some dying prematurely.
"An average of 10 to 12 hours from decision to admit a patient until they are transferred to a bed."
It asks the Prime Minister "as a matter of urgency" to consider supporting strategies to reduce crowding in A&E departments, including a "significant increase" in social care funding to allow patients who are fit to be discharged and a review of the number of hospital beds that are available for acute care.
Video report by Emily Morgan
The letter, which was leaked to the Health Service Journal, describes how they often see 50 patients at a time waiting for a bed in emergency departments, with patients forced to sleep in clinics as makeshift wards.
The NHS has "insufficient hospital and community beds and staff of all disciplines especially at the front door to cope with our ageing population's health needs," it adds.
"As you will know a number of scientific publications have shown that crowded emergency departments (ED) are dangerous for patients. The longer that the patients stay in ED after their treatment has been completed, the greater is their morbidity and associated mortality.
"Recent media coverage has reported numerous anecdotal accounts of how appalling the situation in an increasing number of our emergency departments has become. These departments are not outliers. Many of the trusts we work in are in similar positions."
Video report by ITV News correspondent Geraint Vincent
The A&E heads said they felt "compelled to speak out in support of our hardworking and dedicated nursing, medical and allied health professional colleagues and for the very serious concerns we have for the safety of our patients".
The letter ends with the note that they sign it on behalf of themselves and their departments, but this does not necessarily represent the views of their individual trusts.
A Department of Health and Social Care spokeswoman said in response to the letter: "There has been a 68.7% increase in the number of A&E consultants since 2010, and the NHS was given top priority in the recent budget with an extra £2.8 billion allocated over the next two years.
"But we know there is a great deal of pressure in A&E departments, and we are grateful to all NHS staff for their incredible work in challenging circumstances.
"That's why we recently announced the largest single increase in doctor training places in the history of the NHS - a 25% expansion."