Britain is baking in the hottest April day since 1949, triggering warnings for London Marathon runners amid predictions Sunday's race could be the warmest on record.
Temperatures soared to 27.9C (82.2F) in Northolt, Greater London, shortly after midday on Thursday, making it the warmest April day for 69 years, the Met Office said.
The mercury hit 20C in the Preston area, bringing out people to the city's parks and green spaces. And beaches in Blackpool were busy with folk catching the best of the rays with temperatures around 17C.
And it could get up to 23C (73.4F) in the capital on Sunday, prompting marathon organisers to add more water, ice and shower stations along the 26.2-mile route.
Conditions may be especially difficult for fancy-dress runners, including the almost 100 attempting Guinness World Records dressed in outfits like a suit of armour, a Paddington Bear costume and ski boots.
Regular runners are being advised to consider dropping their goal-times and run more slowly.
Adam Smith, who is attempting the record for fastest race dressed as a mythical creature, told the Press Association: "My costume is particularly warm, with a nice and furry gnome hat and a toadstool to carry.
"The plan is to go on as normal but I've asked if I can have short sleeves - I'm very conscious that there's a world record and then there's being safe.
"I suspect there won't be a lot of people doing PB's (personal bests)".
The 45-year-old lawyer, who has run eight marathons, is raising money for muscular dystrophy research, having lost three family members to the disease.
The hottest London marathon on record was 22.7C (72.9F) in 1996, the Met Office said.
Marathon event director Hugh Brasher said: "Most importantly, runners should adjust their goal for Sunday and plan to run at a slower pace.
"If they were planning to run in fancy dress, they should think carefully if that is appropriate in the forecast conditions."
"We want to stress that there will be plenty of water available and runners should drink according to their thirst and use spare water to douse their head and neck."
Met Office meteorologist John West said: "Potentially we're looking at highs of 23C but we could potentially see some showers coming in from the west which could take a degree or so off.
"Showers could be a bit of a double-edged sword because it could make it that little bit more humid for runners."
There will be 39 first aid stations dotted along the route that runners should visit if they feel overly tired or unwell, organisers said.
Water is available every mile from miles three to 25, while six shower stations can be run through to cool down at miles nine, 13, 17, 20 and 22.