Heatwave conditions are leaving hospitals struggling as doctors walk out in a dispute over pay.
NHS boss Amanda Pritchard said the situation posed “a serious risk to patient safety” as the UK sees temperatures top 30C this week.
The 72-hour walkout in England by the British Medical Association runs until 7am on Saturday.
NHS England issued a statement asking people to check on elderly friends and relatives and warning people with conditions such as asthma are at increased risk.
Speaking at the NHS Confed Expo conference in Manchester, Ms Pritchard said: “We must prioritise the management of what is a serious business continuity incident and therefore a serious risk to patient safety.
“However you feel about the rights and wrongs of industrial action in the NHS our number one priority must be patient care and patient safety.”
She added: “In recent days there have been real pressure on systems not least the heat and demand from industrial action as well. Many teams have been under real pressure from big increases in demand.”
Around 200,000 planned appointments are expected to be disrupted this week as A&Es and “life and limb” care are prioritised.
A junior doctor who is considering moving to Australia for better pay has described working for the NHS as "unbearable".
BMA strike rallies were staged by junior doctors as part of the BMA action in Birmingham, Oxford, Manchester and London.
Dr Sumi Manirajan, 29, a junior doctor from West Croydon and a BMA committee member, said: "It used to be that winter was horrific and the rest of the year was normal service.
"Now it's winter all year round. It's become unbearable working for the NHS.
"Now I'm even considering leaving for Australia. My pay would be fair there.
"Doctors don't want to go but we're being forced to leave the NHS."
The British Medical Association (BMA) is calling for "full restoration" of junior doctors' pay, which it says has been cut by 26%. The Government has offered 5% to end the dispute.
Health Secretary Steve Barclay has refused to re-enter pay talks as the BMA is still planning strike action. The BMA says it will not halt planned action until it gets a “credible offer”.
Mr Barclay tweeted: "The BMA’s 72 hour strike is extremely disappointing and puts patient safety and work to cut waiting lists at risk.
"At recent meetings with the BMA's junior doctors committee we made a fair and reasonable opening offer and were discussing both pay and non-pay issues until they chose to end the talks.
"If the BMA cancels these damaging and disruptive strikes and shows willingness to move significantly from their position, we can resume confidential talks and find a way forward, as we have done with other unions.
"People should attend appointments unless told otherwise by the NHS, continue to call 999 in a life-threatening emergency and use NHS 111 online services for non-urgent health needs."
Shadow health secretary Wes Streeting told the conference: “The junior doctors’ strike this week is likely to be one of the most disruptive in the history of the NHS.
“More than half a million patients have already seen their operations or appointments cancelled as a result of industrial action.
“Waiting lists that should be falling by now continue to rise.
Yet the Government continues its disastrous approach of refusing to negotiate with NHS staff.”
Chris Hopson, chief strategy officer for NHS England, told the conference: "In the end, the decisions around what is paid to junior doctors are a matter for the Government.
“Our job is to make sure that we appropriately manage the consequences of any industrial action.”
From June 19, the BMA will be re-balloting junior doctors to extend its mandate on industrial action and said it could go on until March 2024 or beyond.
Dr Chris Morris, a BMA deputy chairman for Yorkshire, was one of around 15 medics on a picket line outside Leeds General Infirmary.
He said there "won't be any staff left working in the NHS" if the Government continued its "chronic underfunding and not valuing staff".
"The Government needs to recognise the value of junior doctors.
"It (moving abroad) is something I've definitely considered, along with many of my colleagues."
He added that junior doctors "feel like we have been forced into this position" after ministers "did not approach (pay negotiations) in good faith".
Members of the Royal College of Nursing (RCN) in England are voting on whether to extend their mandate on strikes until the end of 2023.
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