Around 25,000 Paramedics, drivers, technicians and call handlers from the Unison and GMB unions across England and Wales are on strike for 24 hours over a row in pay.
However, the call handlers will not strike for longer than 12 hours and call handlers will only strike for six-hour periods.
Patients are being warned to expect “much slower” response times for 999 and 111 calls as there will be fewer ambulances out.
In the South-West, South-East coast, North-West, South-central area, North-East, East Midlands, West Midlands, Yorkshire and Wales, more than 10,000 GMB union ambulance services are also expected to strike.
Patients have been told to still dial 999 for life threatening emergencies and are told to use 111 for anything that is non-urgent, NHS England have said.
Health and Social Care Secretary Steve Barclay said, “Today’s ambulance strike is an unwelcome return to unnecessary disruption and comes at a time when the NHS is already under huge pressure from Covid and flu.
“While we have contingency plans in place, including support from the military, community first responders and extra call handlers, to mitigate risks to patient safety, there will inevitably be some disruption for patients with fewer ambulances on the road.”