Thousands of patients are facing disruption as junior doctors stage their latest walkout in a row over contracts.
More than 5,100 procedures and operations have been postponed due to the 48-hour strike, which started at 8am.
Junior doctors have pledged to provide emergency care cover.
It is the fourth walkout by the British Medical Association over contracts that stop junior doctors claiming extra pay for day shifts on Saturdays in return for a basic pay rise.
Almost 25,000 procedures have been cancelled so far as a result of the dispute.
Dr Anne Rainsberry, national incident director for NHS England, said: “We’ve already seen that a 48-hour strike puts considerably more pressure on the NHS and it’s deeply regrettable that thousands of patients are still facing disruption because of this recurring action.
“As always, the safety and care of patients is our number one priority and everything possible is being done to make sure patients will still be able to access urgent and emergency services.”
The Department of Health called the strike “irresponsible and disproportionate”.
But Dr Johann Malawana from the BMA said: “We deeply regret any disruption this action will cause to patients, but it is because we believe this contract would be bad for the delivery of patient care in the long term that we are taking this action.
“By imposing a contract that junior doctors have no confidence in and refusing to re-enter talks with the BMA, the Government has left us with no choice.
“We want a contract that is fair for all junior doctors – not one which the Government has admitted will disadvantage women – and ensures that they feel valued and motivated so that the NHS can retain the GPs and hospital doctors of the future.”
Strikes planned for 26 and 27 April will see the full withdrawal of labour by junior doctors between the hours of 8am and 5pm.
Health Secretary Jeremy Hunt is facing a second legal challenge – from NHS staff campaign group Just Health – to try to block the imposition of the contract.