Hospital bosses have expressed concern about keeping patients safe as they struggle to secure cover for overnight junior doctor shifts during strikes.
NHS Providers warned that the 96-hour junior doctor walkout will cause a “very long, difficult week” for the health service.
The health service’s top doctor warned that the situation will “become more challenging”.
NHS England’s national medical director Professor Sir Stephen Powis said: “Staff are working incredibly hard during this unprecedented period of industrial action, and we expect the situation to become more challenging each day this strike progresses.
It comes as it emerged that some doctors at a hospital in Weston-Super-Mare have been asked to return to work due to patient safety fears.
Under NHS contingency plans, hospital leaders can request for doctors to return to work for a limited time in certain circumstances “to maintain safe patient care”.
The British Medical Association (BMA) posted on Twitter: “Protecting patient safety during strikes has always been a priority to the BMA. However, poor planning by local management has left the Emergency Department and acute medicine at Weston General Hospital exposed.
“As a result, the BMA has agreed that a total of 7 junior doctors can be asked to volunteer to return to work today and tomorrow.”
Health Secretary Steve Barclay said the timing of the strike was “regrettable”, and he accused the BMA of putting patients at “greater risk” after not agreeing any national exemptions for strike action for some services, such as cancer care.
The Government and the union appeared to be at a stalemate after Downing Street insisted there will be no talks unless junior doctors abandon their starting position of a 35% rise and call off the strikes.
The 35% demand is “completely out of step with pay settlements in other parts of the public sector” and would cost £2 billion, the spokesman added.
Miriam Deakin, director of policy and strategy at NHS Providers, which represents NHS trusts, said: “Keeping patients as safe as possible, trusts’ number one priority, will be even harder than in previous strikes, so it’s all hands on deck.
“Getting through today is just the start. Trust leaders are worried about securing adequate cover for the night shifts ahead. This is going to be a very long, difficult week for the NHS.”
It has been estimated that some 350,000 appointments and operations have been rescheduled as a result of the action.
Mr Barclay said: “I deeply regret these strikes and in particular the timing, which been timed deliberately coming straight after Easter, the fact that the BMA junior doctors have asked their members not to tell NHS managers whether they intended to go on strike or not – making contingency planning more difficult – and also their refusal to agree on any national exemptions.
“The junior doctors committee has refused any national exemptions and obviously that puts patients at greater risk, but we’re working very hard to mitigate those impacts.
“We are ready to have discussions with them but clearly a demand for 35% – over £20,000 for some junior doctors – is not fair or reasonable.”
Dr Vivek Trivedi, co-chairman of the BMA’s junior doctors committee, told the PA news agency: “Where we go forward is, of course, reserving the right for further industrial action if the Government doesn’t negotiate with us, but that’s all we want them to do.
“We just want them to come to the table in an honest and meaningful way.
“The Government has not even tried to meet us anywhere in the middle, it hasn’t given us a counter-offer at all.”
A spokesperson for University Hospitals Bristol and Weston NHS Foundation Trust, which runs Weston General Hospital, said in a statement: “The safety of patients is our top priority.
“As part of our established operational planning for periods of industrial action we’ve been exploring all avenues for ensuring sufficient cover for key services over the course of the week.”
Doctors on picket lines across England described how they are struggling to afford groceries and borrowing money to pay rent.
Meanwhile, hundreds of medics marched past Downing Street and the Houses of Parliament after a rally in central London.
Rebecca Lawson, 43, from West Sussex, said her diagnostic procedure to determine the cause of severe stomach pain had been cancelled twice due to strikes.
The former NHS dental nurse said: “It’s frustrating because you think, ‘we’re paying for this to get away from the disruption of the NHS and we’re still being impacted’.”
The BMA has claimed junior doctors in England have seen a 26% real-terms pay cut since 2008/09 because pay rises have been below inflation.
It has asked for a full pay restoration that the Government said would amount to a 35% pay rise – which ministers have said is unaffordable.
The union said junior doctors can earn as little as £14.09 per hour in their basic pay packet.