Parts of the UK saw temperatures hit almost 24C (75.2F) on Wednesday despite a cloudy start to the day.
In Weybourne, north Norfolk, the mercury had reached 23.9C (75F) during the afternoon, with the temperature across the south-east and London expected to rise higher into the evening.
Forecasters had said there was a 10% chance the UK could see its hottest March day on record.
The nation’s hottest ever March temperature of 25.6C (78F) was recorded in 1968 at Mepal in Cambridgeshire and the Met Office said the mercury is expected to rise to similar levels on Wednesday.
Met Office spokesman Grahame Madge told the PA news agency: “So far the hottest we’ve seen is 23.9C in Weybourne, but in other areas that could top out to perhaps 24-25C by the end of the day.”
It comes as a 14-year-old boy died after getting into difficulties at Goit Stock waterfall in Cullingworth, West Yorkshire, on Tuesday evening, police have said.
Officers have asked for a group of males who tried to help to come forward so they can piece together what happened.
West Yorkshire Police said on Wednesday: “Emergency services were called just after 6pm yesterday evening to reports of a concern for safety.
“A 14-year-old male had entered the water and got into difficulty.
“Emergency services attended, including fire service, ambulance and underwater rescue teams and recovered a body from the water.
“Police are appealing for witnesses of the incident and in particular, there was a group of males in the area who tried to help the teenager and police would urge them or anyone in the area to come forward and assist with the circumstances of the incident.”
The force urged the public to “think again” about the dangers of swimming in lakes, reservoirs, rivers and canals.
The Royal National Lifeboat Institution (RNLI) also reminded people to “take care” if they visit the coast.
RNLI water safety manager Sam Johnson said: “Although the rollout of our lifeguard service starts this weekend, they can’t be everywhere, so people need to think about their own safety and what they would do in an emergency.
“Coastal areas provide a great opportunity to enjoy fresh air and open space but it is important to remember it can be an unpredictable and dangerous environment, particularly during spring and early summer when air temperatures may be warm but water temperatures remain dangerously cold, increasing the risk of cold water shock.”
Swathes of the UK had a cloudy start to Wednesday, with the temperature reaching into the low 20s in London and parts of central and eastern England.
Mr Madge had earlier said: “There is a chance we could see our hottest March day on record today, we’re predicting a 10% chance that it could happen.”
People have made the most of the warmer conditions after Monday’s easing of coronavirus rules, which means groups of up to six, or two households, are now able to socialise in parks and gardens and that outdoor sports facilities can reopen.
Adam Jones, from Moseley, Birmingham, said he saw a “couple of thousand” people convened in Cannon Hill Park, to the south of the city, while out walking with his girlfriend on Tuesday evening.
“It has been noticeably busier since restrictions eased but yesterday was really busy,” the 27-year-old said.
“While there were a fair few people in small groups respecting distancing, they were overshadowed by lots of big groups, sometimes up to 20 to 25 people, blaring music.
“We did one lap of the park and by the time we were leaving it looked like a festival.
“How it looked this morning was disgraceful, so much rubbish that a council worker estimated it would take the whole team most of the day to clear.”
Superintendent Farooq Sheikh, of Birmingham East police, said: “We had officers at Cannon Hill Park yesterday following social posts suggesting that an organised gathering was planned.
“Officers initially found no breaches of coronavirus legislation, with people enjoying the weather in small, individual groups.
“Later on in the evening, the numbers increased and music and lighting equipment had been set up.
“We had a number of complaints from residents about traffic building up around the area.
“The crowds were good natured and engaged with officers, and the park was all clear by 11.30pm.
“Officers did not need to issue any fines.
“We will have extra police officers in the area today, who will be able to use our powers to quickly break up any gatherings that put people’s health at risk.
“We want to reinforce that it is really important to remember that, while people can now meet up with five other people outside, large-scale organised events are still against the law.”
Woodhouse Moor park in Leeds was also “packed” during Tuesday evening, with large amounts of waste such as cans, bottles and boxes left on the ground on Wednesday morning.
Sophie Meredith, 26, who is studying in the city, said: “I walked around the park rather than through it because it was so busy and there wasn’t much or any social distancing going on.
“It’s really disgusting that people left their litter piled up by the bins and across the grass, and it’s an issue that happens every time there’s a hot day in Leeds.
“Today was the worst I’ve ever seen it.
“It’s probably made worse by the fact that everyone’s been cooped up for a year and there are not enough bins in that park.”
Further footage shows London’s Hyde Park similarly littered on Wednesday.
Communities Secretary Robert Jenrick urged people to enjoy the sunshine in a “sensible, cautious” manner by sticking to the rules.
He told ITV’s Good Morning Britain: “We just need to exercise caution and be sensible and pragmatic about how we do that.
“I think the vast majority of people are and will do that, they will enjoy the sunshine this week and at the Easter weekend, but they’ll do so in a sensible, cautious way.
“The two things aren’t mutually exclusive – you can go and out, have fun, enjoy the great outdoors and the sunshine, while also trying to be careful, sticking to the rules and trying to avoid the most crowded places.”
Health Secretary Matt Hancock also tweeted, saying: “Let’s enjoy the sun but let’s do it safely.
“We have come so far, don’t blow it now.”
However, parts of Northern Ireland and Scotland will remain cloudy and wet, with temperatures barely hitting the mid-teens, Mr Madge said.
Temperatures will then drop to the low teens in the east of England from Thursday.
Mr Madge said a “cold front” will start moving down from the north, bringing with it colder, windier conditions in stark contrast to the previous “warm southerly winds”.
Wet weather is forecast for Easter Sunday, with the chance of sleet or snow in Scotland and the far north of England.
Mr Madge added: “Friday should mark a rather malign start to Easter, with colder conditions for large swathes of the country as part of an Arctic plunge.
“Temperatures will be dragged down quite a bit from the day before, with wintry showers expected in Scotland and the most northern regions in England.”