The public health service in England was on Tuesday braced for "the most disruptive industrial action" in its history as junior doctors walk out over pay and working conditions.
The National Health Service (NHS) has been hit by waves of strikes in recent months as soaring inflation eats into wages and pandemic backlogs coupled with staff shortages increase workloads.
But the four-day walkout by junior doctors threatens to be the most serious action yet, with the national medical director of NHS England Stephen Powis warning it will pile "immense pressures" on the service.
"This is set to be the most disruptive industrial action in NHS history, and the strikes tomorrow will bring immense pressures, coming on the back of a challenged extended bank holiday weekend for staff and services," he said on Monday.
"Emergency, urgent and critical care will be prioritised but some patients will, unfortunately, have had their appointments postponed."
Up to a quarter of a million appointments could be postponed, according to the NHS Confederation, which represents the whole healthcare system in England, Wales and Northern Ireland.
Family doctors are reported by British media to be closed for appointments for up to a week, as GPs are drafted in to provide cover for junior doctors.
The NHS Confederation's Matthew Taylor told Sky News the strikes "are going to have a catastrophic impact on the capacity of the NHS".
The service has been "preparing extensively" for the strikes, said Powis, but added that the task was made "much more difficult" due to the "sheer numbers" of appointments that need to be rescheduled.
The walkout will begin at seven am (0600 GMT) on Tuesday and run until the morning of April 15.
Health Secretary Steve Barclay said "unrealistic" calls for a 35 per cent pay rise had led to a breakdown in talks.
"This demand is widely out of step with pay settlements in other parts of the public sector at a time of considerable economic pressure on our country," he wrote in the Sunday Telegraph.
"I recognise their hard work and dedication. But it is deeply disappointing that this industrial action has been timed by the British Medical Association (BMA) Junior Doctors Committee to cause maximum disruption to both patients and other NHS staff."
Mike Greenhalgh, deputy co-chair of the BMA's Junior Doctors Committee, told BBC One's Breakfast show on Sunday: "It's hard to negotiate when only one side is doing it and we're not getting anything back from the government on that front.
"We're happy to meet at any time. We would still meet him over the bank holiday weekend before the industrial action next week," he added.
Junior doctors have up to eight years of experience working as hospital doctors and makeup around half of all NHS doctors, according to official figures.
The UK has been hit by strikes across broad swathes of the economy, with workers struggling with a cost-of-living crisis caused by double-digit inflation.
