England is basking in sunshine with the hottest July day ever recorded.
The Met Office said a temperature of 36.7C (98F) had been reached at Heathrow - breaking the previous record set in 2006.A level 3 "heatwave action" heat-health alert has been declared for all parts of England.
But in Scotland forecasters warned of thunderstorms, torrential downpours and hail stones up to 1cm in size.
Public Health England advised avoiding the sun up to 15:00 BST.
Wimbledon spectators - who have been covering their heads with umbrellas, newspapers and towels - were advised to wear hats.
The centre court roof has been closed to protect the playing surface and people from the heat.
London has been hotter than Rome and Athens, according to figures collated by the Met Office, but not hotter than Paris, which reached 39C (102F), and Madrid, which reached 37C (99F).
As hot as it gets
38.5C (101.3F)
Hottest temperature ever recorded in the UK - Faversham, Kent in August 2003-
36.2C (97.16F) Recorded at Heathrow at 14:50 BST on Wednesday
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7 degrees This much above average counts as "very hot" - 6 degrees over is just "hot"
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5 days Length of time the max daily temperature must exceed average to count as a heatwave
- Affected routes include Abellio Greater Anglia trains between London Liverpool Street and Ipswich and First Great Western services between London Paddington to Bourne End and Henley-on-Thames
- Virgin's East Coast line between Leeds and London Kings Cross is also running a reduced service
- Commuters have been advised to check their journeys before travelling and to carry water and sunscreen
Keeping cool in the heat
Health warnings have also been issued to young children, the elderly and those with serious illnesses who are encouraged to stay in the shade, wear loose clothing and drink plenty of water.
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."
Dr Angie Bone, head of extreme events at Public Health England, said: "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."
Some schools have cancelled their sports days, including Castledon School in Wickford, Essex, and Christchurch Primary School in Ilford, north-east London.
People have been reminded to take care swimming in open water, after the death of a man believed to have drowned in a reservoir in Suffolk on Tuesday.
The heat has caused bin collections to be delayed across Bath and North Somerset because the circuit boards in the compactors of the bin lorries are overheating.
BBC Essex hired an ice cream van to give free ice cream to listeners, paid for by the station presenters and editor.
Breakfast presenter James Whale said: "It's going to be baking in Essex and our listeners are so brilliant we thought they deserved a treat."
Pumpkin, a grey seal, has learned to jump on the sun canopy over his pool, which he uses as a hammock as he rolls around sunbathing.
He learned the trick in May but has been doing it every day recently because of the hot weather.
Hot weather can be fatal for many animals, and the RSPCA has reminded people never to leave a dog alone in a car on a warm day, even with the windows left open.
The PDSA has advised that pets should always have access to shade, and hutches should not be left in direct sunlight.