Top NHS doctors have issued a stark warning to the Prime Minister that patient safety is under threat due to NHS budget constraints.
A letter, which has been signed by more than 2,000 GPs, specialists and consultants, says that medics have "unacceptable levels of safety concerns for our patients".
The doctors said they are "constantly" failing to meet patient expectations and are left "exasperated" because they are unable to provide excellent care.
The letter, published in the British Medical Journal, calls on the Government to increase healthcare spending to at least 10% of the nation's gross domestic product (GDP) in order to meet demand.
" It is impossible to provide effective efficient patient-led innovative healthcare which is free at the point of contact when we spend less on healthcare than other comparable OECD countries," the authors said.
The letter - organised by Anita Sugavanam, consultant anaesthetist, and Rob Galloway, an emergency medicine consultant, at Brighton and Sussex University Hospital - also reiterates calls for a specific ring-fenced budget for social care.
"We understand fully that these are times of austerity and the national budget has never been under more scrutiny," it states.
"However, the rising ageing population and the squeeze on NHS and social care funding are very real.
"We have reached unacceptable levels of safety concerns for our patients within the NHS and simply cannot continue.
"This is the letter we hoped we would never have to write."
It adds: " We are constantly failing to meet our own and our patients' expectations.
"We feel handcuffed and paralysed working in this current NHS.
"We are exasperated and feel demoralised because we are not able to provide and develop the excellent care we were trained to give.
"We are simply fighting fires on a daily basis.
"There is a real risk of a brain drain at our level from the UK if this government does not listen to us."
Commenting on the letter, Dr Mark Porter, chairman of council at the British Medical Association, said: " The entire profession has been calling out for help over the last few weeks but these requests seem to have fallen on deaf ears.
"The BMA has repeatedly raised concerns, both publicly and with the Government, that the current NHS funding settlement is inadequate to deliver the standard of care which patients deserve and that doctors and our colleagues in health and social care want to be able to provide.
"Instead of outlining a plan to deal with the crisis, the Government has tried to play down the pressure that services are under.
"The Government cannot continue to stick its head in the sand.
"Our hospitals are in the red, patients are suffering and staff are working under impossible conditions.
"The system is at breaking point and it's not just our hospitals that are facing these issues - GPs are conducting millions more consultations every year while also facing a recruitment crisis.
"The Government must urgently look at the long-term funding, capacity and recruitment issues facing the system as a whole if we are to get to grips with the pressures the NHS faces year in, year out, but which are compounded during the winter months."
A Government spokesman said: "We are committed to the NHS - that's why we have invested £10 billion in its own plan to transform services and improve standards of care, and recently announced almost £900 million of extra funding for adult social care over the next two years to tackle the pressures of our ageing population.
"Furthermore, the NHS is now carrying out record numbers of treatments with more doctors and nurses providing safer, more personal care than ever before."