The walkout on Wednesday is the latest in a series of strikes this year by "resident" or junior doctors, who make up nearly half the medical workforce and say their pay has been eroded over more than a decade.
The government says it can't afford to increase pay more than it already has, and health minister Wes Streeting has called the strikes "self-indulgent, irresponsible and dangerous".
The industrial action adds pressure to an already-stretched healthcare service.
National Health Service England warned last week that hospitals were facing a "worst-case scenario" from a surge in cases of a virulent strain of flu.
"Staff will come together as they always do ... but sadly more patients are likely to feel the impact of this round of strikes than in the previous two," said Meghana Pandit, National Health Service England's national medical director.
Flu hospitalisations in England surged by more than 50 per cent in early December, reaching an average of 2660 patients a day - the highest for this period. Health leaders have said there was still no peak in sight.
NHS England said there may be fewer doctors on duty than usual during the strikes, requiring staff to prioritise life-saving care, meaning patients with less urgent conditions could face longer waits. Some appointments will be rescheduled.
Shortly after being elected last year, the Labour government agreed a 22 per cent pay rise for doctors to end a dispute that began under the previous Conservative government.
However, after being awarded another 5.4 per cent rise this year, the conflict flared again.
The British Medical Association, which represents resident doctors, is seeking a 29 per cent increase this year to restore pay to 2008 levels in real terms.
A last-ditch government bid to avert the strikes by offering better training options was overwhelmingly rejected by BMA members.
