Temperatures are set to soar to as high as 35C in Britain - amid fears that the heat could disrupt rail services.
Commuters are advised to check their journeys before travelling and have been warned that there may be disruption caused by the hot weather.
The heat is predicted to be so intense that Network Rail has imposed a speed restriction on some services today to stop the rails from buckling.
“As rails are made out of steel, they expand as they heat up and are subject to strong compression,” Network Rail said on its website.
“This expansion has to be managed to reduce the risk of track buckling.
“If the track does buckle, the line must be closed and the track repaired before services can resume, causing considerable disruption.
“Usually, these repairs can’t be done until the temperature of the rails has dropped.
“If a section of track is judged to be at risk, we introduce local speed restrictions - slower trains exert lower forces on the track and reduce the chance of buckling.”
The mercury is predicted to hit 30C (86F) today and 35C (95F) on Wednesday, making Britain hotter than Brazil’s Rio de Janeiro and Bucharest in Romania.
First Great Western services in the Thames Valley area will be disrupted from midday to 8pm.
A First Great Western spokesman said: “Network Rail has advised that, due to the anticipated exceptionally hot weather, there will be speed restrictions to protect track points in the Thames Valley region on Tuesday 30 June 2015.
“As a result there will be no fast trains between London Paddington and Bourne End or Henley-on-Thames from 12pm until 8pm.
“Trains from London Paddington will terminate at Twyford for a connection with a branch line train to Henley-on-Thames, which will remain on the branch line.
“Trains from London Paddington will terminate at Maidenhead for a connection with a branch line train to Bourne End, which will remain on the branch line.”
The heatwave is being caused by a warm front and tropical continental air mass from Europe pushing across the country, bringing high temperatures and humidity.
A health warning has been issued amid concerns that lives could be at risk.
Vulnerable groups including the elderly, young children and people with breathing difficulties have been urged to stay cool as the hot weather pushes across the UK from Europe.
Dr Angie Bone at Public Health England said there could be more deaths than usual.
She said it seemed “likely” that the Met Office would issue a level 3 heatwave alert - which requires community support for at-risk groups, media alerts about keeping cool and a review of safety at public events. She added: “It is possible that we will see an excess mortality but it is too early to tell.
“We know that high temperatures do have an impact on health, particularly on older people and young children and people with chronic diseases.”
However, severe thunderstorms are predicted to strike western and northern parts of Britain by tomorrow afternoon, and temperatures will dip on Thursday - but are likely to rise again before the weekend.