Paul Grundy, chief medical officer at University Hospital Southampton NHS trust, said the emergency department was under “substantial pressure”.
“It’s extremely hot in that environment, it’s extremely busy and we’re asking people please don’t come to the emergency department unless you absolutely need to with life- or limb-threatening illness,” he added.
NHS Hampshire and Isle of Wight said healthcare services were experiencing high demand, as well as heat impacts on buildings and equipment.
It said both South Central Ambulance Service and the Isle of Wight Ambulance Service were dealing with a higher number of calls.
Dr Charlotte Hutchings from the NHS regional body thanked people for their patience and support during the extreme heat.
She said: “By taking precautions, choosing the most appropriate NHS service for your needs, and looking out for those who may be more vulnerable, you are helping to reduce pressure on health and care services.”
A Met Office red warning for extreme heat is in place for Thursday and Friday.