Highs of 35degC are expected on Thursday and 37degC on Friday.
Porthmadog in North Wales had held the record for the hottest day of the year so far, with temperatures reaching 33C (91.4F) on June 28.
Meanwhile sun worshippers in the United Kingdom have sizzled in the hottest temperature of the year so far, with a scorching 33.3C recorded in England yesterday.
Forecasters said there is a chance of hitting 34C or 35C, which could put Britons in line for the hottest day since the mercury hit 34.5C at Heathrow on June 21 a year ago.
Several places have had 54 consecutive dry days since 30 May including a few that have had less than 1mm of rain in the entire 54-day period - the longest spell since 1969, when 70 days passed with no significant rainfall, according to the Met Office.
This would be the hottest July day ever recorded in the UK.
The joint warning from the NHS, Public Health England and the Met Office says there is a 90% probability of heatwave conditions until 09:00 BST on Friday.
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Met Office forecasters suggested that this week the country could experience its hottest July day ever - beating the previous record of 36.7C, measured at Heathrow in 2015 - and even the hottest day on record, exceeding the 38.5C at Faversham in Kent on August 10, 2003.
After weeks without rain, torrential downpours are due to hit parts of Britain, with as much as 30mm (1.2in) expected to fall in just an hour.
Hot weather, especially when prolonged, with warm nights, can have effects on people's health and on certain infrastructure.
"Large hail and strong, gusty winds will be additional hazards".
The heat health watch warning is created to make local services aware that these conditions are being met, and for them to take action.
The level 3 warning, one behind a national emergency, has been issued with temperatures set to be in the high 20s this week.
The heat health watch warning is created to make local services aware that these conditions are being met, and for them to take action. So keep an eye on friends and family who may be at risk. "Also try to carry water with you when travelling".