The Met Office's warning came as employers were called on to dial down their dresscodes to help their workers stay cool.
The weather warning added: "Bouts of rain, heavy and thundery at times, will move in from the west to affect parts of southern Scotland and northern England from the early hours of Wednesday. It will also feel warm and humid overnight".
The outlook for Thursday, Friday and into the weekend is slighter fresher and cooler but mainly dry and bright, with some rain expected on Saturday.
The Met Office is predicted some disruption, most likely from Wednesday afternoon, including a chance of localised flooding and disruption to power supplies due to lighting.More news: London tower block fire death toll reaches 79
Level one: This is the minimum alert and is in place every year from 1 June until 15 September, which is the period that heat-health alerts are likely to be issued.
The hot weather has triggered an amber Public Health England (PHE) Heat Health Watch for much of England.
Many people enjoy hot weather but there can be serious health consequences from too much heat and vulnerable groups are particularly at-risk in prolonged hot spells.
Spokesman Dr Thomas Waite said: "For some people, such as older people, those with underlying health conditions and those with young children, the summer heat can bring real health risks. If you're able, ask if your friends, family or neighbours need any support".More news: Brexit Secretary David Davis targeting a 'deal like no other in history'
There is very little sun expected on Tuesday - but with likely cloud coverage across the sky, humidity levels are set to rise rapidly.
"Temperatures eventually dropped around 4am but they remained in the high teens and low 20s".
Unusual levels of UV are also being recorded in the United Kingdom at the moment, with the strength of the radiation in some spots being as high as that in Cyprus and Gibraltar.More news: Iran launches missiles into Syria in revenge for attacks
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