NHS leaders have for the first time said junior doctor and consultant strike action is "causing significant disruption and risk to patients".
In a letter to the chair of the British Medical Association (BMA), Professor Phil Banfield, senior figures in NHS England said they were "extremely concerned" Christmas Day levels of cover were "insufficient to ensure appropriate levels of patient safety" across local health systems.
Doctors have been staging a three-day walkout - known as Christmas Day cover - which began on Monday at 7am and coincides with the Tory Party conference.
The senior figures said they were "becoming increasingly concerned" that combined periods of industrial action were "impacting our ability to manage individuals who require time-sensitive urgent treatment, for example cardiac, cancer or cardiovascular patients, or women needing urgent caesarean sections".
"Although we recognise that consultants have been giving six weeks' notice of industrial action, we are anxious this in itself is not sufficient to appropriately maintain safe care for these patients," they wrote.
"This is particularly the case when periods of industrial action occur in close proximity - such as during the last month where two of the four weeks have been impacted."
Responding to the letter, Prof Banfield said repeated planning failures by NHS England had made it harder to organise safe striking.
He wrote: "We have always been open to discussing ways in which together we can maintain patient safety during industrial action, and we communicated this directly to colleagues at NHS England, most recently in a meeting just yesterday."
Prof Banfield called on NHS England to communicate their concerns to Prime Minister Rishi Sunak and urged him to open pay negotiations with junior doctors and consultants.
"As you know, no further industrial action has been called, and it is now incumbent upon the government to come to the table, drop their refusal to negotiate on pay and settle its dispute with doctors," he wrote.
Radiographers are set to join doctors on picket lines at hospitals across England, piling additional pressure on NHS services already disrupted by strike action.