South Central Ambulance Service has declared a critical incident due to extreme pressure.
The news was revealed on social media early on Wednesday morning, after a large volume of callouts the previous day.
The post read: “Our staff are working extremely hard to respond to calls and manage the situation and we continue to prioritise those patients with life-threatening injuries and illnesses.
“Declaring a critical incident means we are able to focus our resources on those patients most in need and communicates the pressure we are under to our patients and health system partners.”
People are asked to only call 999 in a life-threatening or serious emergency.
The service which covers seven million residents across Berkshire, Buckinghamshire, Oxfordshire and Hampshire, has been under significant pressure for several months - having previously declared a critical incident in October.
Mark Ainsworth, SCAS director of operations, said: "We declared a critical incident in the early hours of the morning due to extreme pressures across our services.
"This was related to the level of demand with a large volume of calls being received throughout the day and into the night and increased challenges in releasing some of our ambulances from busy acute hospitals.
"This then impacts our ability to get crews back on the road to respond to patients.
"Our staff and volunteers continue to work extremely hard to respond to calls and incidents and we continue to prioritise those patients with life-threatening injuries and illnesses.
"For some patients whose situation is not a life-threatening or serious emergency, we have been discussing their needs, providing advice and urging to them to make their own way to hospital if they do not require an emergency ambulance response.
"Declaring a critical incident means we are able to focus our resources on those patients most in need and communicates the pressures we are under to our patients and health system partners who can provide support.
"We are asking people to help us at this time by using our services wisely and also utilising other healthcare alternatives including 111.nhs.uk for urgent medical advice and guidance wherever possible and appropriate to do so."