Heatwave health alerts have been issued as the Met Office predicted the longest hot spell in four years could begin on Sunday.
A period of hot weather starting this weekend could last for more than a week, making it the longest since a 15-day heatwave in 2018, forecasters said.
The weather is expected to be warm and dry on Friday, reaching 28C, but a little cooler on Saturday, with the high heat beginning in earnest on Sunday.
High temperatures in southern and central England and Wales are expected to reach 31C on Monday and Tuesday, and could reach the high 30s towards the end of next week, though mid-30s are more likely, the Met Office said.
People should make sure that fridges, freezers and fans are working properly and check in on vulnerable family and friends, health authorities said.
Elderly people and those with heart, lung or circulation problems can be particularly prone to health problems as a result of prolonged hot weather.
Alex Burkill, a senior Met Office meteorologist, said: “It does look like the settled weather is going to stick with us, across England and Wales.
“Temperatures will probably stay in the 30s and there’s a potential that they could rise further and we could be looking at high 30s by the end of the week.
“It’s more likely that they stay in the low, perhaps mid-30s bracket as we go through that week.
“Whilst temperatures may not be record-breakingly high, it is more likely that we have a prolonged period of hot weather as we go through the next week or even longer.
“You have to go back to 2018 for the last time you had a heat event like this lasting for such a long time.”
An official heatwave is declared when temperatures are higher than a threshold level, which is different in different areas of the country, for three consecutive days.
The highest threshold is 28C, in London and the south east of England. In 2020 an August heatwave lasting nine days overall saw temperatures reaching 34C or higher for six consecutive days somewhere in the UK.
High heat contributed to 2,556 deaths in 2020, a 15-year high, with the prolonged nature of the hot spells thought to be one reason they were so deadly.
The UK Health Security Agency issued a “level two” health alert on Thursday for the East of England, South East and London, warning people to shade and cover windows and check the symptoms of heat stroke and heat exhaustion.
A level two alert warns health and social care workers that people might be suffering from the effects of high heat.
Dr Agostinho Sousa, head of extreme events and health protection at UKHSA, said: “We want everyone to enjoy the hot weather when it arrives, but also to check in on their vulnerable family, friends and neighbours to make sure they are prepared for the warm conditions ahead.
“High temperatures are predicted for a prolonged period, so make sure to follow our simple health advice to beat the heat, such as covering windows exposed to direct sunlight and making sure that fridges, freezers and fans are working properly.”
Matthew Killick, director of crisis response and community resilience at the British Red Cross, said: “We’re all looking forward to enjoying some warm weather this summer, but it’s important to remember that heat can be very dangerous, especially for children, older people and those with underlying health conditions.
“Climate change means we’re experiencing longer and more intense heatwaves, but a worrying number of people aren’t aware of the risks around hot weather.
“In England alone, there were more than 2,500 excess deaths in the summer of 2020, and unfortunately it’s predicted that heat-related deaths in the UK could treble within 30 years.”