Ambulances and an animal charity have been inundated with calls as Britain swelters on the hottest day of the year so far.
Temperatures are set to soar to 35C (95F) in parts of the country on Tuesday - making it hotter than swathes of Europe - but the hot weather has led to a surge in emergency calls.
London Ambulance Service said it attended 300 more calls than usual to help seriously ill and injured patients.
LAS medical director Dr Fenella Wrigley said: “We see an increase in calls because people forget to stay hydrated and the heat can exacerbate heart and breathing conditions.
“Older people, the very young and people with pre-existing conditions are more at risk so please check on friends, family members and neighbours who are vulnerable.
“We urge everyone out enjoying the sunshine to drink plenty of water, stay in the shade to keep cool, cover up, carry any essential medication with them and drink alcohol in moderation.”
The RSPCA has also seen a spike in calls from members of the public concerned about dogs left in hot cars, conservatories, sheds and caravans.
Inspector Justin Stubbs said three dogs had already died this summer after being left in cars.
The Met Office has declared a Level 3 heatwave alert and Public Health England is urging people to take care.
Dr Angie Bone, head of extreme events at Public Health England, said: “Now the heatwave has arrived people will likely be out and about more enjoying the summer sun.
“But it’s important to remember that there are some people whose health suffers in hot weather. Older people, those with underlying health conditions and young children may all feel the ill-effects of heat over the coming days.
“We’re urging everyone to keep an eye on those you know who may be at risk this summer. If you’re able, ask if your friends, family or neighbours need any help.”
Temperatures of 30.1C (86.2F) - the hottest so far this year - were recorded at Charlwood, Heathrow and Northolt on Tuesday morning, the Met Office said.
But the mercury will keep on rising, with parts of Britain expected to be hotter than Barcelona later in the day.
Met Office forecaster Alex Burkill said temperatures would be “widely” hitting 30C (86F) and are likely to peak in the North West, London and Oxford.
He said: “I there’s a fairly good chance that we’ll see 35C today somewhere. Yesterday was the hottest day of the year so far and we only reached 30C yesterday, so it is going to be the hottest day of the year so far I can say with a pretty good degree of confidence.
“It’s not going to be the hottest day for too far back - we reached 36.7C (98F) on July 1 last year so it will be the hottest day for over a year, but not the hottest July day ever.”
But the good weather is not here to stay. The Met Office has issued a yellow weather warning for severe thunderstorms, large hail and strong winds which could lead to flash flooding of homes, businesses and transport networks.
“The risk of thunderstorms coming across from Northern Ireland later on this evening, and then spreading across Scotland and then also further south across Wales and northern England, and also western parts of England - a big chunk of the UK - as we go through tonight and into tomorrow.”
Air pollution is also set to reach dangerous levels, according to ClientEarth.
Alan Andrews, lawyer for the environment consultant, said the “spike” in pollution caused by high ozone levels was “bad news” for Western Europe and those who suffer from respiratory illnesses.
He called on the Government to do more to promote public awareness of the risk.
“Without quick and appropriate action to clean up the air, incidents like these will only get more severe, and more dangerous,” he said.
But London’s Ice Bar was experiencing more punters than normal trying to get out of the heat.
Tom Hunter, head of marketing, said he was expecting large numbers to visit the bar - which is kept at minus 5C (23F) - to avoid the heat outside.