The government's heatwave plan has been triggered
A heatwave alert has been issued by the Met Office amid warnings of extreme temperatures over the next few days.
There is a 60% risk of a heatwave for Monday and Tuesday with daytime highs in London reaching 32C. It will remain warm at night.
NHS staff have been warned to prepare for a surge of people suffering from the heat, particularly the elderly.
The Department of Health has also asked people to check up on vulnerable friends, relatives and neighbours.
The Met Office has predicted that around the country daytime temperatures could reach 29-30C, with minimum night-time temperatures of 15-18C.
London, the East of England, South West, South East and the Midlands are the most likely to be affected.
Officials had already said this summer might be warmer than the past couple of years.
With climate change, heatwaves are likely to become more common over the next few decades and the Chief Medical Officer has warned of an increase in deaths in times of hot weather.
In northern France in 2003, a three-week heatwave with extremely high day and night-time temperatures caused 15,000 excess deaths.
A Department of Health spokesman said consecutive hot days could be dangerous for people with heart and respiratory problems and in extreme cases, excess heat can lead to heat stroke, which can be fatal.
Ozone can be a big problem for those with breathing problems but although levels can be high during the day in a heatwave, they drop at night so staying indoors in the middle of the day can help.
"Keeping the home as cool as possible during hot weather and remembering the needs of friends, relatives and neighbours who could be at risk is essential.
"The elderly and those who are ill, are particularly vulnerable during hot weather and the most oppressive conditions occur in our towns and cities."
"Windows should be kept shaded and closed when the temperature is hotter outside than inside.
"People with respiratory problems should stay inside during the hottest part of the day."
If anyone is worried that their home or that of a relative or neighbour is too hot they should contact their local environmental health officer, he added.
Other advice in the government's heatwave plan includes to drink cold drinks like water or fruit juice regularly and avoid tea, coffee and alcohol.
Help the Aged and Age Concern welcomed the advice.
A spokesman said: "It is very important that older people look after themselves in the warm weather.
"Older people, especially those on medication, can often find coping with the heat particularly difficult."