More than 125,000 operations and appointments have been cancelled due to next week's junior doctors' strike, as NHS England warned ambulance trusts may need to put on temporary treatment centres.
As the NHS prepares for the first full walk-out in the history of the health service, patients are being told to expect delays during a time of "heightened risk".
Thousands of junior doctors across England will go on strike for two days starting at 8am on Tuesday. They will not provide emergency care as in previous strikes.
NHS England has told hospitals they must focus on "essential services" such as A&E, maternity, resuscitation, major incident plans and mental health crisis intervention.
According to new data from NHS trusts, 12,711 planned operations have been cancelled as a result of the action, including 4,187 inpatient operations and 8,524 day case procedures, all originally set for April 18 to May 2.
Some 112,856 outpatient appointments have also been cancelled over the same period and will need to be rearranged.
This is on top of almost 25,000 procedures and thousands more appointments that have been cancelled as a result of previous strikes.
NHS England said the health service was "pulling out all the stops" to minimise the risks during the walk-outs, which will run from 8am to 5pm on Tuesday and Wednesday.
All trusts have reported that they have plans in place to provide essential services, although services may be "staffed differently" and there may be delays and changes, NHS England said.
Dr Anne Rainsberry, national incident director for NHS England, said it was an "unprecedented situation" and other parts of the health service have been drafted in to help.
Arrangements are in place to recall doctors from the picket line in an emergency, more GP appointments will be available and NHS 111 has increased the number of calls it can take.
NHS ambulance trusts have also been asked to look at providing extra support, including providing temporary treatment centre facilities.
Dr Rainsberry said: "The NHS exists to care for and treat patients and it is with enormous regret that we find patients put in this position.
"We have focussed our efforts on essential services including emergency care but the effects of this action will be felt far and wide, with thousands of people having their operations postponed and their care disrupted for which we sincerely apologise.
"NHS organisations have tried and tested plans to deal with a range of disruptions to seek to ensure continued safe services for patients, which is always our top priority. The NHS has been pulling out all the stops to minimise the risks to the quality and safety of care but this is an unprecedented situation during a time of heightened risk.
"In some places the NHS may be under specific pressure."
Earlier this week, the BMA said it would call off the forthcoming strike if Health Secretary Jeremy Hunt lifted his threat to impose the contract on junior doctors.
But Mr Hunt said it was not possible to "change or delay" the introduction of the controversial contract.
The Academy of Medical Royal Colleges (AMRC) has called on Mr Hunt to accept the BMA's offer and return to the negotiating table.
A BMA spokeswoman said doctors wanted to protect patients and had given the NHS notice of the industrial action.
She said: "Junior doctors deeply regret disruption to patients but they are taking this action because they fundamentally believe the Government's plans will be bad for patient care in the long term.
"Crucially, there is still hope this action can be avoided. The BMA has been clear that it will call off next week's action if the Health Secretary removes the threat of imposition and returns to negotiations."
A Department of Health spokesman said: "Well over a hundred thousand patients have now been directly affected by the BMA's extreme and irresponsible action, which even its own junior doctor leader advised against.
"We have continually sought a negotiated solution over three years of talks, during which there were two walkouts from the BMA, and now there's only the one issue of Saturday pay outstanding. If the doctors' union had agreed to negotiate on that as they promised to do through Acas in November, we'd have a negotiated agreement by now. Instead, we had no choice but to proceed with proposals recommended and supported by NHS leaders -- which were 90% agreed with the BMA."