Thousands of NHS operations and appointments are being cancelled and rescheduled as a result of Wednesday’s strike, the UK governments have said.
In England, around 60,000 non-urgent operations, out-patient appointments, tests and follow-up appointments are thought to be postponed.
This is based on the NHS doing 20% less work if around a fifth of staff go on strike.
In Scotland, at least 3,000 operations and thousands more hospital appointments are being rescheduled.
Patients have been sent letters warning them of the disruption. Hospitals say emergency and critical care will be unaffected.
The Department of Health in England said it was expecting at least 5,500 non-emergency procedures 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.
Lancashire Teaching Hospitals is postponing tests and operations such as X-rays, some heart procedures, appointments in its orthopaedic assessment and treatment centre, cardio respiratory service and specialist mobility and rehabilitation clinic.
Sheffield Teaching Hospitals has warned patients that it will be cancelling all outpatient clinics and non-urgent operations.
And Trafford Healthcare Trust has warned it will need to cancel some non-urgent appointments and surgery or treatment.
On an average day, 28,000 patients have planned treatments or operations in England and there are 60,000 diagnostic tests.
In Scotland, 1,000 procedures due to take place in Lanarkshire are being rescheduled, meaning appointment numbers have been reduced to less than 30% of what would normally take place.
Greater Glasgow and Clyde has postponed 1,000 routine hospital admissions and day cases, as well as a “significant number” of outpatient and community appointments.
Across the UK, the NHS will operate as if it were a weekend or Bank Holiday.
The focus will be on emergency and critical care and patients with the highest levels of need such as chemotherapy and kidney dialysis.
Calls to 999 will still be responded to, but ambulance staff will undertake fewer duties such as transporting patients to hospital.
Figures suggest around 400,000 nurses and healthcare assistants, paramedics, physiotherapists, and support staff like cleaners and administrators, will join the action against changes to public sector pensions.
The Royal College of Nursing, the Royal College of Midwives and the British Medical Association are not taking part in the strike but are likely to support the protests in other ways.
The NHS is not allowed, by law, to bring in agency workers to cover staff on strike.
Health secretary Andrew Lansley said: “We have to put patients first. The NHS and those working in the health service should never take action that would prejudice the interests of patients.
“It is especially irresponsible to do so when there continue to be active discussions with the trade unions.
“These discussions are genuine, in which I have been personally involved. The improved offer the Government put on the table three weeks ago directly reflected requests that the trade unions had made.
“We continue with discussions and hope to come to an agreement by the end of the year.”
Unison’s head of health Christina McAnea said members did not take strike action lightly.
“Most of them will first and foremost ensure that there is adequate cover is in place and that patients’ safety is not compromised.
“But by the same token they will be wanting to demonstrate their opposition to what the Government is trying do to their pensions.”