Patients "will suffer" as a result of the impending doctors' strike, the medical regulator has warned as it urged junior doctors to seriously consider whether or not to take part in the action.
In a new guidance for junior doctors ahead of the strike scheduled for next week, the General Medical Council (GMC) said that in order to avoid patient harm "the right option may be not to take action that results in the withdrawal of services".
The rolling action - the first of its kind in the history of the NHS - will see junior medics withdraw labour, including emergency care, for a week each month until the end of the year.
But the GMC said the scale of the action at such short notice cannot be justified.
Medics will stage strikes from September 12 to 16, on October 5, 6, 7, 10 and 11, November 14 to 18 and December 5 to 9, withdrawing from labour between the hours of 8am and 5pm, in the ongoing dispute over a controversial new contract.
The Government and British Medical Association (BMA) remain at loggerheads over the contract, which the Department for Health says will provide a seven-day NHS.
The GMC guidance states: "Given the scale and repeated nature of what is proposed, we believe that, despite everyone's best efforts, patients will suffer.
"In light of this, the right option may be not to take action that results in the withdrawal of services for patients."
It is a doctor's responsibility to "put their patients first and protect them from harm", GMC chief executive Niall Dickson said.
Professor Terence Stephenson, chairman of the GMC, said he was "extremely concerned about the impact which this prolonged campaign of industrial action will have on patients' care and on the public's trust in doctors".
He added: "The BMA's announcement marks a substantial escalation of the previous industrial action in that it involves: the removal of emergency care (as well as routine appointments); a rolling programme of action of indefinite duration; the removal of junior doctors' services for five days of each month (rather than one-off days of action); and much shorter notice to NHS employers of the first bout of action which leaves little time to prepare.
"The health service is under huge pressure. During previous industrial action all doctors went to considerable lengths to make sure that patients continued to receive a good and safe level of care.
"We know that doctors will again want to do their utmost to reduce the risk of harm and suffering to patients. However, for the reasons given above, it is hard to see how this can be avoided this time around.
"To suggest otherwise would be a disservice to the enormous contribution made by doctors in training to the care and treatment of NHS patients every day.
"We therefore do not believe that the scale of action planned at such short notice can be justified and we are now calling on every doctor in training to pause and consider the implications for patients."
Prof Stephenson added: "We understand that the Government and the BMA have held talks about the small number of remaining issues and we hope that talks can resume in order to avoid plunging the health service into a further crisis which is in no-one's interests."
Mr Dickson added: "Parliament has not fettered the right of doctors to take industrial action, unlike some other professions and occupations. Doctors therefore have a right to strike and take industrial action.
"The question each doctor must ask, however, before taking action is whether what they are proposing to do is likely to cause significant harm to patients under his or her care or who otherwise would have come under his or her care. This is a matter of professional duty and we expect each doctor to comply with it.
"This advice is issued under the authority of the 1983 Medical Act which governs the behaviour of all doctors practising in the UK. The Act and the accompanying guidance require doctors to exercise their professional responsibilities in the interests of their patients, to put their patients first and protect them from harm.
"The GMC has powers under the Act to investigate and apply sanctions to any doctor whose behaviour has fallen consistently or seriously below the standards required. Where we are presented with evidence that a doctor's actions may have directly led to a patient or patients coming to significant harm, we would be obliged to investigate and if necessary take appropriate action."
The advice for doctors contemplating industrial action adds that doctors must take "reasonable steps" to satisfy themselves about the arrangements made to cover their shifts while taking action.
During the strikes, if medics are asked to return to work "in good faith" by their employers they must do so.
But in separate guidance to doctors in leadership roles, it says that striking doctors should only be asked to return to work if there are "genuine and significant concerns" about patient safety.
Six strikes have already taken place across England during the lengthy dispute, causing disruption to hundreds of thousands of patients who have had appointments and operations cancelled.
In May it looked as though a breakthrough had been reached in the dispute after both sides agreed to a new deal.
Then in July, the Government announced that it would impose a new contract after junior doctors and medical students voted to reject the contract brokered between health leaders and the BMA.
The BMA said it will call off the strikes if the Government agrees to stop the imposition.
A BMA spokesman said: "Patient safety remains doctors' priority and since the announcement last week of further action, the BMA has been liaising with NHS leaders so that plans can be put in place swiftly to minimise disruption for patients.
"This action is still avoidable. The BMA has said it will call off next week's action if the Government puts a halt to plans to force junior doctors to work under a contract they have rejected because they don't believe it is good for the future of patient care or the profession."
The Department of Health said it would not comment specifically on the latest GMC guidance.
But a spokeswoman provided a previously distributed statement which reads: "As doctors' representatives, the BMA should be putting patients first not playing politics in a way that will be immensely damaging for vulnerable patients.
"What's more, the BMA must be the first union in history to call for strike action against a deal they themselves negotiated and said was a good one.
"Whilst there are many pressures on the frontline, funding is at record levels, with the highest number of doctors employed in the history of the NHS. Co-operation not confrontation is the way forward to make sure patients get the best treatment and the NHS is there for people whenever they need it."