About 2,000 senior NHS doctors have written to Prime Minister Theresa May to urge her to increase spending on health and social care.
The letter – read and signed by NHS consultants, associated specialists and GPs – is due to be posted from Brighton at 14:00 GMT on Thursday.
It reads: “We have reached unacceptable levels of safety concerns for our patients within the NHS.”
A government spokesman said it was committed to investing in the NHS.
The letter was organised by emergency consultant Dr Rob Galloway and consultant anaesthetist Anita Sugavanam, both doctors at the Brighton and Sussex University Hospital NHS Trust.
The clinicians said it was a letter they hoped they would “never have to write”, but they feared a “brain drain” from the health service.
“We are constantly failing to meet our own and our patients’ expectations,” they wrote.
“We apologise to them and we also empathise with them. 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.”
The letter calls on the prime minister to increase spending and to ring-fence budgets for social care and the NHS.
The clinicians wrote: “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.”
A spokesman for the Department of Health 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 aging 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.”
A week of coverage by BBC News examining the state of the NHS across the UK as it comes under intense pressure during its busiest time of the year.