Thousands of junior doctors across England are preparing for a second day of strike action over a row with the Government about a new contract.
More than 5,000 operations and procedures across England have been cancelled ahead of the 48-hour strike which began at 8am on Wednesday.
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 during the strike, with two further 48-hour strikes planned from 8am on April 8 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.
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.
Dr Anne Rainsberry, national incident director for NHS England, said: "Unfortunately the cumulative effect of these recurring strikes is causing disruption to thousands of patients, for which we can only apologise.
"A 48-hour stoppage puts considerably more pressure on the NHS.
"The impact of the action so far is broadly in line with what we were expecting but we know that the second day is going to be more difficult and have made sure plans are in place to respond to any rising pressures."
Urgent and emergency care services will be available as normal but hospitals are expected to be under extra pressure.
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 attracts 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 too many 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.
"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."
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."