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The Coastguard had its busiest day for more than four years on Friday as the UK recorded its third-hottest day ever.
Many calls were about people being cut off by the tide, missing children and swimmers getting into difficulty.
The total number of UK incidents was 329, including 232 callouts for coastguard rescue teams, 129 for lifeboats, 22 requiring aircraft and three for a hovercraft.
Some local authorities have warned that beaches are becoming “unamanageable” due to large swathes of visitors.
It comes as temperatures reached 37.8C (100F) at London’s Heathrow Airport and 37.3C (99.1F) at Kew Gardens, in west London, the Met Office said.
Julie-Anne Wood of HM Coastguard, said: “Yesterday was a beautiful day weather-wise in much of the UK. It was less beautiful a day for those who got themselves into trouble and had to be rescued. Some people will remember July 31 for all the wrong reasons.
“We completely understand that people want to enjoy the coast. We also know that even the most experienced swimmer, paddleboarder and walker can be caught out by currents and tides respectively.
“We’re heading into some more good weather and we would really ask you to check and double check the tide times – put a timer warning on a smartphone to remind you – be aware of things like rip currents, and make sure you have a means of contacting us if things do go wrong.
“As the figures show, we’ll always respond when someone calls 999 and asks for the Coastguard, we’ll always answer distress on VHF (radio) and we’ll always do everything we can to rescue those in need. All we ask in return is that you take extra care at the coast – it can be unmerciful to the unwary and even to those who know it well.”
The weather saw large crowds flock to the beaches on Friday and sparked fears about how daytrippers, who basked in the sunshine after months of lockdown, would keep to social-distancing measures.
Brighton & Hove City Council tweeted on it was “concerned about the number of people in the city”, and stated that “large numbers make it impossible to maintain physical distancing”.
Thanet District Council asked people to avoid four of the area’s beaches, including Margate’s Main Sands, due to the number of visitors.
Leader of the council Cllr Rick Everitt told the PA news agency: “Early on, it became clear that they were going to reach levels of which we were concerned about.”
Authorities expressed concern about keeping people safe in the water, as well as the potential spread of Covid-19 and maintaining social distancing.
“The RNLI only have a certain capacity,” Mr Everitt said.
“They’re on seven Thanet beaches this summer, which is slightly fewer than usual, and they’re doing a great job where they are but they don’t have unlimited resources to deal with people in the water.
“If you have too many people on the beach, it just becomes unmanageable from that point of view.”
The hottest day ever recorded in the UK was registered at 38.7C (101.7F) at Cambridge University Botanic Gardens last year. The second hottest day was on 10 August 2003, when temperatures rose to 38.5C (101.3F) in Faversham, Kent.
The Met Office said temperatures will be back closer to average on Saturday, but it will still be relatively warm in the south east.
Met Office forecaster Simon Partridge said “the weekend is going to be pretty good”, and added: “Both days are going to be mostly dry across the UK with sunny spells and some showers at times that may be out to the north and the west of the country.”
Temperatures in the South East on Saturday could reach 26C or 27C and it could feel a little bit fresher with highs of 23C or 24C on Sunday.
The RNLI and coastguards are also telling people not to take inflatables to the beach and urging them to be aware of the tide times, local hazards and potentially deadly rip currents.
The warning comes as seven people were rescued after being caught in a strong, fast-moving current in Wales on July 26, according to the RNLI – whose lifeguards dealt with more than 1,500 incidents involving rip currents last year.