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 not to be “caught out” by soaring temperatures, with the UK expected to be hotter than some of Europe’s top holiday destinations, including Ibiza and Tenerife.

The mercury is set to rise to 37C (98.6F) in London and the South East on Friday, the second day of a heatwave expected to last until at least Sunday.

Forecasters believe Friday could even surpass the 37.8C (100.04F) recorded in Heathrow on July 31 – the hottest day of the year so far, and the UK’s third warmest ever.

But the Met Office has warned that after a slightly cooler week people could underestimate the heat, which could cause dehydration and sunburn, especially among the most vulnerable.

Grahame Madge, a Met Office spokesman, said: “Everyone needs to be careful during this heatwave, especially on Friday, where we are going to see a dramatic rise in temperature in some parts, exceeding the heatwave threshold.

“After a slightly cooler week, people should make sure they aren’t caught out. If you need to travel, keep hydrated and apply sunscreen; the chance of sunburn and dehydration will be much higher.”

The threshold for a heatwave is three days of temperatures over 25C (77F) across most of the nation and 28C (82.4F) in London.

Public Health England (PHE) has issued a heat-health warning, with people advised to stay cool indoors by closing curtains that face the sun and ensuring pets or children are not kept in vehicles.

Ishani Kar-Purkayastha, consultant in public health at Public Health England, said: “This summer, many of us are spending more time at home due to Covid-19. A lot of homes can overheat, so it’s important we continue to check on older people and those with underlying health conditions, particularly if they’re living alone and may be socially isolated.”

Elderly people are among the most vulnerable to hot weather, with advice telling them to contact neighbours if they are living alone, to try to stay indoors during the afternoon and to carry a bottle of water when out.

Caroline Abrahams, charity director at Age UK, said: “We want older people to continue to enjoy the warm weather but, if it becomes uncomfortably hot, we advise some sensible precautions, particularly for anyone who has breathing problems or a heart condition.

“It’s a good idea to remain indoors during the worst of the heat during the day. It’s also advised to wear thin, light clothing, drink plenty of fluids and to eat normally, but perhaps more cold food than usual, particularly salads and fruit which contain a lot of water and help us stay hydrated.

“We know that extreme heat can aggravate lung and heart conditions so our advice is to take care and if you are breathless, even after you have rested, to seek medical advice.”

Motorists looking to travel to seaside locations have been urged to ensure cooling systems are filled to the correct level, to look at the coolant date and to not overload their vehicle with luggage.

Breakdown experts Green Flag has predicted just under 127,000 breakdowns to occur between Friday and Wednesday, which translates to 15 breakdowns every minute.