Britain is preparing itself for the hottest day of the year today, with temperatures possibly rising to 34C (93F).
The mercury rose to 30.5C (88F) on the mainland and a sweltering 33C (91F) on the island of Jersey yesterday, but forecasters warned it could hit 34C in the South East today.
Health chiefs urged employers to allow staff to travel outside rush hour to avoid overcrowded public transport during the exceptionally hot spell, now entering its third day.
Meanwhile drivers on one of Britain's busiest motorways faced delays of more than an hour in the heat after a serious accident left one carriageway closed.
Public Health England (PHE) urged people to look out for those who may feel unwell in the hot weather.
Dr Angie Bone, head of extreme events at PHE, said the heat could be dangerous for older people, young children and those with serious illnesses, and urged employers to be flexible.
She 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. For those working or exercising outdoors, strenuous physical exertion during the hottest part of the day should be kept to a minimum."
A car and a van were involved in an accident on the M25 between junction 9 at Leatherhead and junction 8 at Reigate in Surrey, Highways England said.
All lanes on the eastbound anti-clockwise carriageway were closed, the agency said, with delays of around 60 minutes and queues back to at least junction 10 at Cobham.
Police and officers from Highways England were at the scene and motorists left their cars to seek shade on the hard shoulder.
The AA warned drivers to make sure they had plenty of drinking water and advised open top motorists, particularly with bald heads, to apply suncream or a cap.
Edmund King, AA president, said: "Many drivers complained that road closures on the M25 truly turned it into the road from hell. We often advise drivers to be prepared in cold weather but it is equally as important to be prepared in extreme hot weather. Drivers need to take plenty of drinking water and ensure that their cars are prepared to idle in hot weather by checking coolant levels.
"Window blinds can help shield children and passengers from extreme sunlight and drivers should have decent sunglasses. Open top drivers, particularly with bald heads, should apply suncream or a cap. In these hot temperatures children or dogs should never be left in parked cars."
The warm conditions at Wimbledon forced officials to reduce capacity to allow fans to cope.
The mercury hit 29.3C (84.7F) in SW19, as it did commonly around the country, and a Wimbledon spokesman said: "We have reduced today's initial capacity slightly to 38,000 to allow people more room in the grounds and therefore more space in the shade and easier access to the free water points.
"The daily capacity is dependant on the number of courts in operation and the prevailing weather conditions so it would not be the first time we have regularly adjusted the daily capacity."
Yesterday's highest mainland reading of 30.5C was at Northolt, west London, while the reading of 33C in Jersey was in St Helier.
The highest recorded July temperature was at Wisley, Surrey, in 2006, where the mercury peaked at 36.5C (98F).
The heat has also caused problems for rail services, with train companies having to cancel journeys or slow them down after 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.
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.
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.
Matt Martin, forecaster at MeteoGroup, said the high temperatures could trigger thunderstorms in the Midlands and North today.
Conditions are expected to be cooler tomorrow before rising to 30C again on Saturday, before falling back into the early 20s for next week.