Britain faces its hottest June day ever for a third day in a row today with highs of 38C.It comes as the unprecedented heat causes unusual scenes including swimmers defying the rules at Hampstead Heath and huge queues for air conditioning units.Today is also a record-breaking third consecutive day of red extreme heat warnings and follows spectacular lightning displays as thunderstorms hit the UK overnight.Videos filmed across the country including in Manchester, Edinburgh and Somerset showed extraordinary storms lighting up the night sky in the early hours of today.A yellow warning for thunderstorms and up to 20mm (0.8in) of rain had been issued for swathes of England, Scotland, Wales and Northern Ireland from midnight to 10am.London and Manchester could reach 36C today, with Belfast and Cardiff facing highs of 26C. It comes after another 'tropical night' when temperatures failed to fall below 20C - with an overnight low in Herstmonceux, West Sussex, of 22.7C last night. It comes after a new June record of 36.7C was set yesterday in Merryfield, Somerset – just a day after another new record on Wednesday of 36.1C in Gosport, Hampshire; which itself beat a longstanding record of 35.6C dating back to summer 1976.Yesterday was also the UK's seventh hottest day ever, matching a record set in 1911 in Northamptonshire, which had stood as the country's hottest day for nearly 80 years. Thousands of schools and nurseries have shut during the heatwave - with some asking parents to pick children up early today amid soaring classroom temperatures - and a South East Water hosepipe ban was brought in for Kent amid surging demand.Transport services have faced major disruption with rail operators urging people to only make essential journeys and one telling people not to travel for beach trips.At Hampstead Heath, where the ponds offer a place to cool down, rule breakers have sparked fury by turning wildlife havens into their own private diving ground.Ponds preserved for hundreds of years to protect nesting ducks and swans and their habitats in north-west London have been invaded by impatient swimmers not wanting to pay a small fee or book online to use mandated swimming spots just yards away. Swimmers break the rules by swimming in Hampstead Heath's model boating pond on Tuesday Officials said there were dozens of swimmers in the pond at Hampstead Heath on TuesdayTwo huge 'No swimming' signs at Hampstead Heath which were seen in place yesterday Wardens approach people dipping their feet in the pond at Hampstead Heath on May 28The culprits are all ages and genders, according to two security guards hired by the council to man the crowds of up to 50 people who swarm the pond after work at 5pm.Two huge 'No swimming' signs are not enough to deter crowds of sun seekers from spoiling the nature by diving in. Security guards stand watch all day but say they are powerless to actually catch people without the ability to hand out fines or penalties.Now, council officials at the City of London Corporation have even put up barbed wire around the pond's perimeter in a bid to physically bar people from entering the water.A video obtained by the Daily Mail which was taken on Tuesday, when temperatures hit 34.6C in South-East England, showed around 20 people swimming into the pond and using the water pumps as flotation devices. HOTTEST DAYS IN UK HISTORY BEFORE THIS WEEK'S HEATWAVE 1 40.3C July 19, 2022 Coningsby, Lincolnshire 2 38.7C July 25, 2019 Cambridge 3 38.5C August 10, 2003 Faversham, Kent 4 38.2C July 18, 2022 Pitsford, Northamptonshire 5 37.8C July 31, 2020 London Heathrow 6 37.1C August 3, 1990 Cheltenham, Gloucestershire 7= 36.7C July 1, 2015 London Heathrow 7= 36.7C August 9, 1911 Raunds, Northamptonshire 9 36.6C August 2, 1990 Worcester 10 36.5C July 19, 2006 Wisley, Surrey It follows the pond invasions during the May heatwave, which saw crowds splash into the protected habitat while young ducklings and cygnets could be seen swimming desperately away nearby.Two security guards on duty yesterday, when the record for the hottest June day was broken for a second day in a row, told the Daily Mail they have been threatened by people when they confront them.The officers, who wished to remain anonymous, said: 'On Tuesday there were around 30 to 40 people at one time in the model boating pond - how can we possibly tackle all of them? We can only question them and tell them to move on.'They often get aggressive and try to fight us. They actually come up to us and say 'we see you every day, we're watching you, trying to intimidate us'.'They added: 'We can't give out fines, we can just report it to our officer at the Hampstead Heath Constabulary but the authorities know about it. But we need four or five officers to actually tackle it.'People are still not listening, they stand here after one hour and carry on. It's all ages all genders that do it, you'd be surprised. It's not just young people.'The guards said usually one ringleader jumps in, is applauded by others, and then other groups follow. They added that while there are good people who do not harm the wildlife, these are 10 per cent of people, while 90 per cent will break the rules.But other people told the Daily Mail they felt very differently. One young couple called the booking system and allocation of proper swimming ponds 'bureaucracy' and said people should be free to swim in public spaces.The woman said: 'Bureaucracy has ruined the nation - we shouldn't have to book things in advance to enjoy a public space. It should be a first-come, first-served basis instead of booking everything. A lightning storm in the distance behind The Needles on the Isle of Wight just after midnight A lightning storm over Stonehenge in Wiltshire early this morning, pictured just after midnight Lightning strikes in a thunderstorm over Weymouth in Dorset last night, seen from Portland Lightning strikes over the Hillsborough area of Sheffield this morning as thunderstorms hit Sunrise this morning on Glastonbury Tor in Somerset as the extreme heatwave continues'I don't agree with the gendered spaces anyway too - we walked past the men's pond and there's not that many people in it. And apparently theirs is better which isn't fair.' The young man added: 'Public places we are allowed to swim in are not accessible. It should be free, it should not cost anything. There should also be more space for people for free that you don't need to book.'Devoted fisherman and retired artist, David Paul, 66, told the Daily Mail he worried about the environmental effects. He said: 'It was really bad last year. 'I remember because people were swimming across to the little island there and you've got a shallow bank that goes a third of the way out.'I was fishing on the platform and saw people hanging onto the pumpheads, so I would've thought that would be dangerous but they use them as floatation devices. It's bad for wildlife especially when they go out to there, obviously it's bad for fish as well.'He explained: 'When they stir up the bottom it's been undisturbed for a long time there's stuff at the bottom that should not be coming to the top. Like blooms and things.'Fishing is fine on the bottom as it doesn't disturb the birds. But last year, there was a nest destroyed which was sunk by swimmers who shoved it to the bottom. With the men's pond is right next door it doesn't seem necessary.'The City of London Corporation's open spaces are patrolled by dedicated constabularies, including the Hampstead Heath Constabulary. Alderman Gregory Jones KC, chair of the Hampstead Heath, Highgate Wood and Queen's Park Committee, said: 'Our Lifeguarded Bathing Ponds, Lido, and the wider Heath are enjoyed by millions of people every year.'We are asking visitors to follow the rules, respect our staff and fellow visitors, and only swim where it is permitted and properly supervised, so everyone can enjoy the Heath safely.'Londoners were urged to take extra care after thermal imaging technology recorded surface temperatures of up to 65C yesterday on the city's pavements and playgrounds.