The East of England Ambulance Service has declared a “critical incident” in response to “huge pressure”.

The ambulance service, which covers Hertfordshire and nearby counties, is one of three nationwide to sound the alarm today (December 20).

In a statement it said pressure was mounting due to 999 call volumes and hospital handover delays.

“Declaring a critical incident means we can ensure our resources are focused on patients with the greatest need, as well as allow us to access wider support from our health and care partners,” the trust added.

READ MORE: Urgent plea as Watford A&E under 'major pressure'

The North East Ambulance Service said it declared a critical incident on Monday afternoon as a result of “significant delays for more than 200 patients waiting for an ambulance, together with a reduction in ambulance crew availability to respond because of delays in handing over patients at the region’s hospitals”.

South East Coast Ambulance Service said it declared a critical incident “following a period of more than a week of sustained pressure across our 999 and 111 services, significantly impacting on our ability to respond to patients”.

The moves to declare critical incidents come as fears grow over the impact of an ambulance strike on Wednesday, when thousands of paramedics, technicians, control room workers and other staff walk out.