Thousands of junior doctors have gone on strike across England as the dispute with the Government over pay deepens.
The 72-hour walkout by medics runs until 7am on Saturday and is expected to lead to thousands of NHS appointments and operations being cancelled.
They are demanding a 35% pay hike but the Government has offered just 5%.
It comes as NHS chiefs warned the number of people seeking emergency help will rise as the hot weather continues across the UK.
NHS leaders said urgent and emergency care will remain the priority.
NHS England medical director Professor Sir Stephen Powis warned: “We're keeping emergency services as normal - maternity services, trauma - but of course there will be a lot of disruption to regular care."
Around 50,000 appointments a day were disrupted during a four-day strike by junior doctors in April, with the latest industrial action set to have a similar impact.
Sir Stephen called on the Government to publish the long-promised NHS workforce plan.
"For us the long-term workforce plan, which we've been working on at the request of the Chancellor, is really important and we hope that the Government will be publishing that very soon," he told Sky News.
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"It takes a long time to train some staff, particularly doctors, so we need to plan well in advance.
“I've said previously that I want to see the number of medical school places expand."
Shadow Health Secretary Wes Streeting said Rishi Sunak should have time to sit down with striking junior doctors if he has an hour "to sit with Boris Johnson negotiating gongs for cronies".
Asked about the walkouts, Mr Streeting told the BBC: "I think the reason junior doctors are out on strike is because they don't have someone to negotiate with.
"I think the question at this stage, having failed to get to a negotiated settlement, is where's the Prime Minister?
"If he's got an hour of his time to sit with Boris Johnson negotiating gongs for cronies and peerages in the House of Lords, he should have an hour at least to negotiate an end to these terrible strikes which are causing misery for the doctors involved and even more importantly misery for patients who are seeing their operations delayed and cancelled."
But he refused to say whether Labour would bow to demands for a 35% wage rise.
"I've got to be honest and say, not least because of the state of the public finances as they are today - and I really do fear what the state of the economy will be by the time of the next general election - I can't yet be sure what the state of the public finances will be and at the moment I'm fearing the worst,” he said.
"So I'm not going to make promises today unless I can be 100% sure I can keep them after the election".
The strike is the third this year by junior doctors and is expected to cause mass disruption.
There have also been concerns about staffing, with some consultants saying they would not provide strike cover unless employers agreed to a higher overtime rate.
Dr Vivek Trivedi and Dr Robert Laurenson, co-chairmen of the British Medical Association’s junior doctors’ committee, said in a statement: "Junior doctors are in despair at this Government's refusal to listen.
"It should never have taken two whole rounds of strike action to even put a number on the table, and for that number to be a 5% pay offer - in a year of double-digit inflation, itself another pay cut - beggars belief.
"We have made clear that junior doctors are looking for the full restoration of our pay, which has seen a 26% cut.
"Junior doctors in England have seen their pay cut in real terms by more than a quarter over the last 15 years.
"Today they are demonstrating what that means to the survival of the NHS."
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