Hospital managers are planning to postpone thousands of non emergency operations today, because of the public sector unions striking over pension changes.Patients across the UK have been sent letters warning them of the disruption.
Diagnostic tests and outpatient appointments will also be delayed, but hospitals insist emergency and critical care will not be affected.
Managers say they are preparing as they would for Christmas or bank holidays.
An estimated 400,000 nurses and healthcare assistants, as well as paramedics, physiotherapists, and support staff like cleaners and administrators have said they will join the action on 30 November over changes to public sector pensions.
However, the main medical unions – the Royal College of Nursing, the Royal College of Midwives and the British Medical Association are not taking part.
The Department of Health in England said it was expecting at least 5,500 non-emergency procedures like hip and knee operations to be rearranged.
More than 12,000 patients are likely to have diagnostic tests postponed, and 40,000 outpatient appointments are expected to be rescheduled.
On an average day, 28,000 patients have planned treatments or operations in England and there are 60,000 diagnostic tests.
However, managers say they are putting plans in place to make sure people can still get emergency or urgent care, in the way they do on bank holidays or at Christmas.
Patients needing urgent treatment like chemotherapy and kidney dialysis will still be able to get it, and maternity units will remain open.
Calls to 999 will still be answered, but patients are being urged to think hard and only call if it is a genuine emergency.
The Health Secretary, Andrew Lansley, said health service workers should not take action that harms the interests of patients.
“I would ask staff to consider carefully whether going on strike is the right thing to do,” he said.