Forecasters say it could be the warmest day for nine years - and the beach-towel weather will continue into the weekend
Highs of 35C (95F) are anticipated in the South East and into the Midlands - close to the July record of 36.5C (98F) set in Wisley, Surrey in 2006.
The beach-towel weather will continue into the weekend, though Thursday will be slightly cooler.
Britons may also need to dig out their brollies, with thunder spells predicted for some areas later in the week.
A level-three heatwave alert could be issued today - one level below a national emergency.
Public Health England (PHE) warned there could be an increase in illness and deaths.
"While hot weather is enjoyable for most people and uncomfortable for some, sadly experience tells us that exposure to excessive heat can kill, with most cases of illness and death caused by heart and lung disease," Professor Paul Cosford said.
Network Rail was forced to cancel or delay some journeys on Tuesday amid fears tracks could buckle in the heat - and more of the same is expected today.
"There are speed restrictions all over the country due to the hot weather so there will be some delays and some disruption," a spokesman said.
"We apologise to passengers for delays but obviously the restrictions are in place to minimise disruption on the network."
Rail company Southeastern said people might wish to avoid travelling between 1pm and 7pm.
But it is not all bad news for commuters, with PHE recommending flexibility from employers.
Dr Angie Bone said: "During very hot weather, pregnant women and people who have chronic illnesses, including cardiovascular, respiratory, renal conditions, diabetes or Parkinson's disease, may experience discomfort if indoor temperatures are particularly hot and in using public transport.
"Employers should ensure indoor areas are kept cool and consider allowing these individuals to travel to or from their place of work during cooler, or less busy times of the day."
If a level-three alert is issued, social and healthcare services would have to mobilise community and voluntary support for high-risk groups.
Media alerts about keeping cool would also be issued.
The heatwave is being caused by a warm front and tropical continental air mass from Europe.
MeteoGroup have warned that wind is carrying dust whipped up from the Sahara towards Britain, where it could coat cars.
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