The UK could see record-breaking temperatures on Friday, with forecasters saying it could be the hottest day of the year. People are being warned to look out for older people, young children and those with underlying health conditions, as the UK prepares for the heatwave to continue through the weekend.

The Met Office issued the amber heat health warning, the second-highest available, on Thursday as it warned people to look out for each other and drink plenty of fluids, while avoiding excessive quantities of alcohol, to deal with temperatures that could rise as high as 38C (100F) in some places.

“Although much of the UK can expect a spell of warm and sunny weather lasting into early next week, it’s going to turn very hot for parts of England and Wales with temperatures widely reaching above 30C on Friday, Saturday and Sunday,” said the Met office’s chief meteorologist, Dan Suri.

“Friday is likely to be the hottest day with temperatures of 36C to 37C in parts of east and south-east England. It’s possible temperatures could reach similar levels on Saturday, before falling slightly on Sunday.

“There’s also a small chance temperatures could reach close to 38C in one or two spots in the south-east on Friday, but this will partly depend on the chance of cloud spreading in from the south-west during the afternoon. Along with hot weather by day, it will stay warm and humid overnight with temperatures remaining in the high teens and low 20s celsius.”

Forecasters are advising people to “close curtains on rooms that face the sun to keep indoor spaces cooler and remember it may be cooler outdoors than indoors”.

The RNLI urged families heading to the beach to be aware of potential dangers.

Last week, on 31 July, the UK recorded the hottest day of the year so far and the third-warmest on record, with the mercury rising to 37.8C (100.04F) at Heathrow airport. The coastguard reported its busiest day for more than four years as it dealt with more than 300 incidents.

Ahead of this week’s sunny spell, Gareth Morrison, RNLI’s head of water safety, said: “Our coastline is a fantastic place to spend time together as a family, especially when the sun is out and it’s hot.

“But there are also plenty of potential dangers, especially for those who aren’t fully aware of their surroundings and may be visiting a particular beach for the first time. We are advising everyone planning a visit to a beach or the coast to follow [the] beach safety advice.”