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."