Scorching temperatures on what is now officially the hottest day of the year are likely to cause travel chaos as the sweltering heat expands rail tracks.
Experts predict that temperatures could rocket to 35C (95F). Just before midday, a temperature of 34.5C was recorded at Northolt in north-west London - the hottest day since 2006. This follows a week of scorching weather as the mercury rose to 30.5C (88F) on the mainland and 33C (91F) on the island of Jersey on Tuesday.
Yesterday, some First Great Western fast train services had to be redirected to avoid vulnerable parts in the track.
Travel disruption is expected to continue today due to the heatwave and passengers are advised to check their journeys before they travel.
First Great Western confirmed that Network Rail's speed restrictions will mean that direct London Paddington to Bourne End services will terminate at Maidenhead from 12pm to 8pm.
A train shuttle service will be in place to carry passengers between Maidenhead and Bourne End.
High Speed Train services are expected to run as normal, however there may be delays as services past through the affected area and some services may be cancelled to protect the evening peak.
A Network Rail video shows that when temperatures soar tracks can buckle, causing delays. Trains are often slowed down in the heat to reduce impact on rails that have expanded and have a higher risk of buckling.
First Great Western will be providing more than 30 extra timetabled train services in both directions between Twyford and Henley to accommodate the Henley Royal Regatta. First Great Western said their plans for the regatta were not heat related.
Passengers on the South Eastern service were advised by drivers this morning to leave work early as trains will have speed restrictions because of the heat this evening.
Many were surprised that the hot weather conditions would affect train travel.
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.
Dr Angie Bone, head of extreme events at Public Health England, 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."
The balmy forecast has triggered a 'Level 2' alert with the Met Office warning: "Heatwaves can be dangerous, especially for the very young or very old or those with chronic disease."
Road travel was not much better yesterday. 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, on Tuesday, 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: "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).
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.