Today will be the hottest day of the year so far as temperatures are predicted to rise above 30C (86F), spelling trouble for rail services and people with respiratory problems.
Sweltering conditions have led rail companies to cancel services as Network Rail warned tracks could buckle in the heat.
Several fast trains from London Paddington were cancelled after Network Rail imposed speed restrictions to protect track points.
The rail operator instructed train companies to slow down at vulnerable locations where tracks could buckle.
Some passengers will have to take slower services from the station to Henley-on-Thames and Bourne End, changing at Twyford or Maidenhead.
A spokesman for the company said: "First Great Western is advising passengers that due to the anticipated hot weather Network Rail is to impose a speed restriction in the London Thames Valley area to protect track points on Tuesday June 30.
"As a result there will be no fast trains between London Paddington and Bourne End or Henley-on-Thames from 12:00 until 20:00."
But he emphasised that only six services out of 1,520 had been affected so far.
Health warnings were also issued for people suffering from lung conditions.
Vicky Barber from the British Lung Foundation Helpline said: "During hot weather, the air we breathe has lower moisture levels than usual, which can have a drying effect on our airways. As a result, people with respiratory conditions such as COPD or severe asthma may find it harder to breathe, feel more tired, or find their lungs feeling heavy or tight."
She recommended that people with lung conditions avoid going outside at midday, wear loose clothing and drink plenty of water.
Other vulnerable groups including the elderly and young children have also 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."
The heatwave is being caused by a warm front and tropical continental air mass from Europe pushing across the country, bringing high temperatures, humidity and possibly Saharan sand.
Paul Knightley, forecast manager at Meteogroup, warned that dust - whipped up from the Sahara - is being carried by the wind towards Britain where it could coat cars.
Temperatures are expected to soar again tomorrow - possibly breaking the record for July.
Highest July temperature ever recorded was measured at Wisley in 2006, where the mercury peaked at 36.5C (98F).