Britain could experience a heatwave next week, the Met Office and the Department of Health said today.
Temperatures could reach 32C (89.6F) in London on Monday and Tuesday, with other parts of the country also enjoying prolonged sunshine.
Night-time temperatures in London will fall no lower than 19C or 20C (66F).
In other regions, daytime temperatures could reach 29C to 30C (86F), with night-time temperatures of 15C to 18C (64.4F).
The East of England, South West, South East and the Midlands will see the highest temperatures outside London.
As a result, the Met Office is triggering alert level 2 of its Heatwave Plan, which means there is a 60% risk of high temperatures in one or more regions on at least two consecutive days and during the night.
A spokesman for MeteoGroup UK, the weather division of the Press Association, said: "We are looking at a hot spell of weather coming in next week.
"The warm, humid air from the past couple of days is going to stagnate over the UK and the near continent, and will be coupled with strong sunshine and increasing humidity.
"It will become quite oppressive in places next week. There is also a growing risk of thunderstorms throughout the week."
The Department of Health said people should take care as temperatures rise over the weekend and into Monday.
A spokeswoman said: "The heat is especially dangerous for the very young, older people or those with serious illnesses.
"In particular, it can make heart and respiratory problems worse. In extreme cases, excess heat can lead to heat stroke, which can be fatal.
"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.
"Elderly people and those who are ill are particularly vulnerable during hot weather and the most oppressive conditions occur in our towns and cities."
The Department advises closing windows and using blinds or curtains when the temperature is hotter outside than inside.
Other tips include:
* People with serious health problems (for example heart conditions), should avoid going out in the heat, especially between 11am and 3pm.
* Drink cold drinks such as water or fruit juice regularly and avoid tea, coffee and alcohol.
* Stay tuned to the weather forecast and plan ahead with supplies - think of elderly neighbours who may need help.
* Keep plenty of water to hand and stay in the shade where possible.
* Identify the coolest room in the house to use as a room to cool down or sleep in.