All of England's ambulance service trusts are on the highest alert level as they try to cope with the pressures of the heatwave and a new spike in COVID cases.
Each of the country's 10 ambulance trusts have confirmed they are at level four of the Resource Escalation Action Plan (REAP), meaning they are under "extreme pressure", as first reported by the Health Service Journal (HSJ).
Ambulances are battling high temperatures with a depleted workforce caused by COVID-inflicted absences.
On Monday evening, South Central Ambulance Service, which covers Berkshire, Buckinghamshire, Hampshire and Oxfordshire, declared a critical incident because of "current pressures on our services".
Watch: Health alerts issued as UK hit with heatwave
In a statement, it said: "We continue to prioritise our response to those patients with life-threatening and serious emergencies but, due to the current levels of pressure we are seeing, there will be delays in responding to other patients with less urgent needs who are assessed as requiring an ambulance response."
The trust asked patients to seek alternative treatment or advice via NHS 111 online or by calling 111.
The statement added: "We are experiencing an increasing number of 999 calls into our service, combined with patients calling back if there is a delay in our response to them. As a result, our capacity to take calls is being severely challenged.
"This is combined with the challenges of handing patients over to busy hospitals across our region and a rise in COVID infections, as well as other respiratory illnesses, among both staff and in our communities.
"This week we are also faced with high temperatures across our region which we know will lead to an increase in demand on our service. All of these issues combined are impacting on our ability to respond to patients."
England's 10 ambulance trusts, North East, Yorkshire, North West, West Midlands, East Midlands, South Western, South Central, South East Coast, London and East of England, are all on level four.
A spokesperson for the London Ambulance Service, which moved to level four on Monday, told Yahoo News UK: “As a result of a sustained demand on both our 999 and 111 services, and with hot weather set to continue over the next few days, we have taken the decision to move up to Level 4 of our Resource Escalation Action Plan (REAP) – which represents ‘extreme pressure’.
“This move allows us to reallocate resources on the road and in our control rooms and to reprioritise our operational efforts to ensure we provide the best care possible to Londoners.
“The public can support us by only calling 999 in the event of a life-threatening emergency and by taking steps to keep hydrated and stay out of the sun at the hottest periods of the day.”
Donna Hay, strategic commander at the North East Ambulance Service, told Yahoo News UK: “As a result of sustained pressure on our service and wider system pressures, as well as anticipated pressure continuing over the next week, including a potential increase in heat-related incidents, we made a decision to increase our operational alert level to four on 11 July.
“The public can continue to support us by only calling 999 in a life-threatening emergency.”
On Monday, Portsmouth Hospitals University NHS Trust declared a critical incident at its Queen Alexandra Hospital in Hampshire.
The trust said: "Due to extreme pressure across the site, an increasingly challenging staffing position and the added strain of the prolonged high temperatures this week, we have taken the difficult decision to declare a critical incident.
"Our emergency department (ED) remains full with patients and we have very limited space to treat emergency patients. We are only able to treat patients with life-threatening conditions and injuries, so any patients who arrive at ED without a life-threatening condition or injury will be redirected to alternative services that can help."
Watch: Amber warning for extreme heat as temperatures soar past 30C