More than 5,000 operations and procedures across England have been cancelled ahead of a 48-hour strike by junior doctors.
Medics will go on strike from 8am on Wednesday in a row with the Government over a new contract.
Health Secretary Jeremy Hunt has announced he will impose the contract on junior doctors - everyone up to consultant level - after months of talks with the British Medical Association (BMA) failed to reach a resolution.
Junior doctors will provide emergency care only on Wednesday and Thursday, with two further 48-hour strikes planned from 8am on April 8 and and April 26.
New figures from NHS England from 228 organisations, of which 154 are acute hospital trust, show that 2,077 inpatient procedures have been cancelled due to Wednesday and Thursday's industrial action alongside 3,187 day-case operations and procedures.
Hundreds more routine clinics and appointments are likely to be affected.
Dr Anne Rainsberry, national incident director for NHS England, said: "This is clearly going to be a difficult couple of days. A 48-hour strike will put significantly more pressure on the NHS and the cumulative effect of these recurring strikes is likely to take a toll.
"The safety and care of patients is always our number one priority and staff across the NHS are doing all they can to minimise the impact on patients of the action.
"We will closely monitor events as they unfold to ensure plans to deal with the pressures are robust and people are ready to respond to any emerging difficulties."
Urgent and emergency care services will be available as normal but hospitals are expected to be under extra pressure, NHS England said.
Where possible, patients are being asked to contact their GP, seek advice from their local pharmacist, call 111 or check the NHS Choices website.
In an emergency, people should still call 999 or go to A&E.
The BMA is currently seeking a judicial review over imposition of the contract, which it says is not acceptable to junior doctors.
The major sticking point has been over weekend pay and whether Saturdays should attract extra "unsocial" payments.
Currently, 7pm to 7am Monday to Friday and the whole of Saturday and Sunday attracts a premium rate of pay for junior doctors.
But the Government wanted the Saturday day shift to be paid at a normal rate in return for a hike in basic pay.
The BMA rejected this and urged Mr Hunt to accept its proposal to reduce the 11% rise in basic pay offered by ministers and instead have better premium rates on Saturdays.
The imposed contract, which is due to take effect in August, has an increase in basic salary of 13.5%.
Under the new arrangements, Mr Hunt said no doctor working contracted hours would see a pay cut while too many night shifts and long shifts will also be limited.
Under the new contract, 7am to 5pm on Saturdays will be regarded as a normal working day.
Doctors working one in four or more Saturdays will receive a pay premium of 30%.
A Department of Health spokesman said: "Patients have so far seen more than 1,9000 operations cancelled as a result of the BMA's irresponsible and unjustified industrial action.
"The new contract, 90% of which was agreed with the BMA, and endorsed by senior NHS leaders, is a very good deal for doctors and the NHS.
"We urge junior doctors to look at the detail of the contract and the clear benefits it brings."
The Royal College of Radiologists said it remains "deeply concerned" about the impact on patients, now and in the future, of the continuing dispute.
A statement said: "We also recognise the severe and continuing stress caused to those of our members who are junior doctors by many months of an increasingly bitter dispute, the negative impact of which will be felt for years to come.
"We are therefore strongly encouraging our training schemes and training scheme personnel to support trainees in every possible way at this most difficult time.
"We have already made known the concerns of our clinical oncology fellows and members over the potential impact of imposition of the new contract on the care of patients with cancer.
"We have also been made aware of factors in the new contract which specifically disadvantage radiology trainees and which will in the future make it harder for trainees with experience in other specialties to enter radiology training.
"For these reasons, we call on both sides in this dispute to step back from the brink for the benefit of patients."
Dr Johann Malawana, the BMA's junior doctor chairman, said: "We deeply regret disruption to patients, and have given trusts as much notice as possible to plan ahead, but the Government has left junior doctors with no choice.
"Ministers have made it clear they intend to impose a contract that is unfair on junior doctors and could undermine the delivery of patient care in the long term.
"The current proposals will affect those already working the most unsocial hours, hitting key parts of the NHS with the greatest problems in attracting and keeping doctors - such as our accident and emergency departments.
"This action is wholly avoidable but the Government must get back around the table and negotiate with junior doctors, rather than simply impose a contract in which they have no confidence."
Lawyers for Mr Hunt have responded to the BMA's proposed claim for judicial review.
In a letter from the Government Legal Department, lawyers argue that the claim that Mr Hunt was in breach of the Equality Act is "misconceived".
They also urged the BMA not to institute "costly" legal proceedings.
The letter states: "You claim that the Secretary of State has acted unlawfully by failing to comply with his Public Sector Equality Duty (PSED) under section 149 of the Equality Act 2010.
"Your claim is misconceived. The Secretary of State has had proper regard to the PSED throughout the process thus far.
"In the circumstances the Secretary of State considers that your client's claim is bound to fail. We note in this regard a report in the Health Service Journal dated 19 February 2016... The article refers to the Claimant receiving legal advice that: (a) there is nothing inherently unlawful about the new contract and, (b) there is no unlawful discrimination in the decision to introduce a new contract. We agree and we note there is no suggestion to the contrary in this pre-action letter.
"We also note, and agree, with the comment by the writer of the article that; 'There will also be questions about the validity of pursuing legal action that will be a drain on taxpayers resources... given its limited chances of preventing the contract imposition'.
"Accordingly we hope that your client will refrain from instituting costly, and ultimately ineffectual, proceedings costly to both the Claimant's members and to the taxpayer. If, however, proceedings are issued, they will be resisted."