The NHS's medical director in England has claimed that the next junior doctors' strike will "irreparably damage" trust in the profession.
Sir Bruce Keogh called the decision to include emergency cover in the strike, planned for 18 hours between 8am and 5pm on Tuesday April 26 and Wednesday April 27, as a "watershed moment" for the NHS.
Writing in the Observer, he said the move ran "against the grain" of the health service.
The British Medical Association (BMA) said junior doctors would be disappointed Sir Bruce was attacking frontline medics.
Sir Bruce said that doctors were "the most trusted profession", and called this a gift.
He said: "By withdrawing emergency cover, we risk crossing a line, which will irreparably damage this trust and the reputation of our profession.
"So I encourage every doctor considering withdrawing emergency cover to dig deep and ask whether such action is fair to patients or compatible with the values and privilege of being a doctor."
He added: "I worry that the withdrawal of emergency cover will put our sickest, most vulnerable patients at greater risk.
"This challenges the ethical framework on which our profession is founded and runs against the grain of our NHS and our personal and professional values."
The BMA representative for junior doctors, Dr Johann Malawana, said none of his colleagues wanted to take the action but they had been "left with no choice" and called for the Government to resume negotiations.
He said: "Junior doctors would rather be in hospital, doing what they do, day in, day out: caring for patients.
"But they have already done everything else in their power to make their voices heard - protests, marches, petitions, emergency care only strikes. By continuing to ignore them, the Government has left them left with no choice."
Dr Malawana added that he regretted any disruption to patients and said that other doctors and staff would provide emergency care for those who need it.