Thousands of junior doctors across England are going on strike in a row with the Government over a new contract.
More than 5,000 operations and procedures across England have been cancelled ahead of the 48-hour strike by junior doctors.
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 trusts, 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 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 come in 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%.