Temperatures over 28C (83F) have been recorded in Surrey, making today the hottest day of the year so far.
Kew Gardens recorded 28.1C (83F) at 1pm, beating the previous high of 27.8C (82F) in Gravesend, Kent, last Friday - and temperatures are expected to rise further.
But the weather has spelled trouble for rail services and people with respiratory problems.
Sweltering conditions led train 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 locations where tracks could buckle.
Some passengers will have to take slower services 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 1200 until 2000."
He emphasised that only six services out of 1,520 had been affected so far.
Health warnings were 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 (chronic obstructive pulmonary disease) 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 were also 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.
The highest recorded July temperature was at Wisley, Surrey, in 2006, where the mercury peaked at 36.5C (98F).