Professor Sir Stephen Powis, NHS England's national medical director, says almost all routine or pre-planned care could be impacted in some way.

Junior doctors in England are due to walk out for 72 hours from 7am on Wednesday in an ongoing dispute over pay, with the strike due to end at 7am on Saturday.

It also comes as health agencies raised concerns about the impact of warm weather on care.

Senior doctors and nurses are due to be drafted in to cover the most necessary work usually undertaken by junior doctors, but that will lead to other work being postponed.

Professor Powis said thousands of routine procedures have already been rescheduled but he called on people to attend any appointments they have if they have not already been told they are postponed.

In April, a similar walkout by junior doctors saw 196,000 hospital appointments and pre-planned operations rescheduled.

Professor Powis said: "The NHS has been preparing extensively for this next set of strikes, but we know that - with the sheer number of appointments that need to be rescheduled - it will have an enormous impact on routine care for patients and on the waiting list, as procedures can take time to rearrange with multiple teams involved.

"As we enter the seventh month of industrial action across the NHS, and as this action becomes more frequent, we are now seeing an extraordinary cumulative impact on our services and crucially on our staff, who continue to go above and beyond to maintain safe patient services during this challenging period.

"Emergency, urgent and critical care will be prioritised this week, but some patients will unfortunately have had their appointments postponed - if you haven't been contacted to reschedule, please do continue to attend your planned appointment.

"This week, as ever, use 999 and A&E for life-threatening emergencies, using NHS 111 online for all other health conditions."

Senior health leaders have meanwhile implored the government and junior doctors to reopen negotiations ahead of the strike.

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NHS Confederation chief executive Matthew Taylor said: "It's disappointing that the NHS is having to brace itself for a further round of industrial action once again as the government and junior doctors have not found common ground on the pay dispute.

"Both sides will need to cede some ground if there is to be any hope of a resolution, and the NHS really needs to see one soon.

"Leaders have prepared well again ahead of the strike and are doing all they can to prioritise patient safety, which will mean that some elective activity and outpatient appointments will take a hit in favour of life and limb care."

Saffron Cordery, NHS Providers' deputy chief executive, added: "We understand the frustration of junior doctors and other staff groups who've seen their pay fall behind inflation and made the difficult decision to strike, but this can't continue.

"We risk sleepwalking into a summer of strikes if talks don't resume as a matter of urgency. The government and unions must re-open talks without delay."

Talks between the British Medical Association and the government broke down in May and the strike was announced after a vote took place in the following days.

Health Secretary Steve Barclay has insisted that ministers' doors remain open, but accused the BMA's Junior Doctors Committee of refusing to shift from its 35% pay demand, despite an intermediary being brought to negotiations.

A Department of Health and Social Care spokesperson responded: "Another three days of strikes by junior doctors will risk patient safety and lead to further disruption and postponed treatments.

"We made a fair and reasonable opening offer to the BMA and were in active discussions about both pay and non-pay issues.

"Unfortunately, it seems they are unwilling to move meaningfully away from unreasonable demands that would see doctors' pay increase by either 35% this year, or at least 49% by next year, which were the two propositions they put on the table.

"We are working with NHS England to put in place contingency plans to protect patient safety. The NHS will prioritise resources to protect emergency treatment, critical care, neonatal care and trauma."