Around 25,000 ambulance workers across England and Wales are striking today (10 January).
Paramedics, call handlers, drivers and technicians from the GMB and unions are striking today across the UK.
Staff are staging 24-hour walkouts over demands for a pay rise above inflation.
NHS England has advised patients to continue to call 999 for life-threatening emergencies but to use 111, GPs and pharmacies for non-urgent needs.
Services in London, the North East, North West, South West and Yorkshire will be affected, with ambulance staff in Milton Keynes not striking.
Health and Social Care Secretary Steve Barclay told Sky News: "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."
Milton Keynes Hospital CEO, Professor Joe Harrison, told MKFM last month: "We are lucky in Milton Keynes that our nurses are not striking and also fortunate that the South Central Ambulance service which looks after our services, the majority are not going to be striking either"
Ambulance staff are set to strike again on 23rd January with nurses taking strike action on 18th and 19th January.
MKFM spoke to MKUH CEO Joe Harrison about strikes, 999 pressure, waiting times and Strep A as part of an exclusive interview. You can listen back here - Professor Joe Harrison, CEO of Milton Keynes University Hospital
READ AGAIN: Milton Keynes Hospital not expected to be affected by nurse and ambulance strikes, CEO tells MKFM
