Thousands of junior doctors across England are on their second day of strike action in a row with the Government over a new contract.
More than 5,000 operations and procedures across England have been cancelled as the 48-hour stoppage enters its second day.
Hospitals are expected to be under more pressure than on Wednesday, with NHS England suggesting there may be more problems discharging patients owing to lower numbers of doctors.
Junior doctors are providing emergency care only until the end of the strike at 8am on Friday. Two further 48-hour strikes are planned from 8am on April 6 and April 26.
On Wednesday, NHS England said that 44% of junior doctors - out of a possible 26,000 working on a typical day - reported for duty on the day shift.
But it confirmed that the figure included doctors who had never intended to strike, such as those working in emergency care.
Health Secretary Jeremy Hunt has announced he will impose the new contract on junior doctors - everyone up to consultant level - after months of talks with the British Medical Association (BMA) failed to reach a resolution.
Figures 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, NHS England said.
Hundreds more routine clinics and appointments are likely to be affected.
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, though Government lawyers have argued this is "misconceived".
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 attract a premium rate of pay for junior doctors.
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 reduce the offer of basic pay and instead have better premium rates on Saturdays.
The imposed contract, which is due to come into force 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 night shifts and long shifts would be limited.
But 7am to 5pm on Saturdays will be regarded as a normal working day.
An Ipsos MORI poll for BBC News released on Wednesday revealed that 65% of the public supported the doctors' cause.
A Department of Health spokesman said: "Patients have so far seen more than 19,000 operations cancelled as a result of the BMA's irresponsible and unjustified industrial action."
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."