Thousands of NHS workers, including nurses and ambulance staff, have begun a four-hour walk out over pay.
Six trade unions agreed to the action, from 07:00 to 11:00 BST, which will disrupt a wide range of NHS services across England.
Urgent and emergency services will remain unaffected, with union bosses saying their members will provide “life and limb” cover during the strike.
A&E units remain open and 999 emergency calls will be answered throughout the action, but hospital outpatient services and community clinics will be affected.
Military personnel and police have been put on stand-by to help ambulance services if required.
The workers are protesting for a 1% pay rise for all NHS staff, which the government claims would be too expensive in the current climate of austerity despite plans for an 11% pay rise for politicians coming into effect next year.
The trade unions involved in the action, which is the first strike by NHS staff over pay in three decades, are Unison, Unite, GMB, the Royal College of Midwives, UCATT, and the British Association of Occupational Therapists