Lives are being put at risk by the crisis in the NHS and social care, senior medics at the Royal College of Physicians (RCP) have warned the Prime Minister.
In a letter signed by RCP president Professor Jane Dacre and 49 members of council, representing 33,000 doctors across 30 specialties, the doctors warned that hospitals were "over-full, with too few qualified staff".
They said they felt "compelled to speak up" as they worked on the front line in the NHS and see patients "waiting longer on lists, on trolleys, in emergency departments and in their homes for the care they need".
They added: "We are treating more patients than ever before.
"This increase in patient need - as our fellow citizens live longer, with more complex conditions - is outpacing the resources we have to care for them safely.
"Our NHS is underfunded, underdoctored and overstretched.
"The ambulances queuing outside emergency departments are a visual testament to the crisis in social care and the NHS.
"Our hospitals are over-full, with too few qualified staff, and our primary, community, social care and public health services are struggling or failing to cope.
"Patients are waiting longer on lists, on trolleys, in emergency departments and in their homes for the care they need."
The letter comes after figures on Tuesday showed that record numbers of people experienced long waits in A&E.
Waits of more than 12 hours among elderly people have more than doubled in two years, according to NHS Digital data.
The doctors said pressures in social care mean more people are being pushed into hospitals and they are trapped there for longer.
"An increasing number of people, although clinically ready to go home, cannot safely leave hospital as the care system is unable to cope.
"People's lives are being put at risk or on hold, affecting families across the country."
The doctors warned that the issues are "demotivating and demoralising" the clinical workforce, with staff threatening to leave.
Money must be put into the NHS and social care to relieve the pressures, they argued, saying the money allocated by the Government was not enough.
They said: "It is essential that we match the demand on health services with the resources to meet it.
"We understand that the current financial pressures mean difficult choices.
"We welcome the decision to increase the number of medical students, and the commitment to extra resources for the health service.
"But front-line staff and managers across health and social care are clear: investment levels are not sufficient to meet current or future patient needs."
Seventy-five leading health and care experts have also signed a letter warning Theresa May that the challenges facing the health and care system will "accelerate" and are calling for "fundamental action" to address the problems.
Dr Mark Holland, president of the Society for Acute Medicine, said the RCP letter provided "a reasoned and balanced account of the problems faced by our health service".
He added: "The intervention of the college at this time reflects the seriousness of the position we have reached.
"The gravitas of our college and its compulsion to act on this matter must make everyone acknowledge we are living in an extraordinary time in the history of the NHS and that urgent action is now required to make things right again."
A Department of Health spokesman said: "We know the NHS is facing increasing demand from an ageing population but this makes building a safer healthcare system more urgent, not less.
"Since just last year, we have 3,100 more nurses and 1,600 more doctors.
"We're also joining up health and social care for the first time and investing £10bn to fund the NHS's own plan to transform services and relieve pressure on hospitals."