Thursday’s 36.4C high in Yeovilton surpassed the 36.1C recorded in Gosport, Hampshire, which broke the longstanding record for June set in both 1976 and 1957.
Meanwhile, as 14:00 BST temperatures had reached 35C in Otterbourne, Hampshire, 34.3C in Wiggonholt, West Sussex and 34C at Bournemouth Airport.
Within the red extreme heat warning area, temperatures could exceed 36C or 37C in some locations on Thursday, accompanied by very high humidity.
The Met Office said people should expect population-wide adverse health affects “leading to serious illness or danger to life”, substantial changes in working practices and daily routine required, high risk of failure of heat-sensitive systems with loss of power, along with transport delays.
A wider amber warning is also in place in Wales, southern England, the Midlands and part of northern England, external, with areas as far north as Manchester expected to see temperatures reach above 30C.
And a separate amber warning for extreme heat, lasting from midnight on Friday, external until 23:59 covers south-east England, eastern England and parts of the Midlands.
At least six NHS trusts in England have declared critical incidents because of the heatwave, sources have told the .
It comes amid reports the extreme heat has caused problems with IT systems, cancer equipment, lab testing and scanners.
London Ambulance Service said it has recorded its highest ever number of life-threatening emergencies as temperatures soared across the capital “driven by the extreme heat” across the capital.