The first smog and heatwave warnings of the summer have been issued, with forecasters predicting Saturday will be the hottest day of the year so far.
Forecasters say London temperatures may reach 31C (88F)
The government said high ozone levels were expected across much of England and in south Wales.
Warning of the first smog of the year, Defra said people should take "sensible precautions" such as avoiding outdoor exercise in the afternoon.
And the Met Office issued a heatwave health warning for northern England.
Forecasters say Saturday will be the hottest day of the year, with temperatures reaching up to 31C (88F) in London.
Defra's smog warning applies to southern England, including the south- east, south-west and Greater London, and to north-east England, Yorkshire and the east and west Midlands.
The Met's first Heat Health warning of the year was issued for north-east England, and the Yorkshire and Humber region.
A spokesman for the office said the much higher than average daytime temperatures of up to 30C (86F) in those regions would put "extra stress" on the body.
This could cause an increased risk of strokes, and problems for people with heart conditions and chronic pulmonary diseases, such as asthma.
The Met Office said: "With many sporting and charity events taking place around the country, the strength of the sun and the combination of heat and humidity will also be important factors to consider."
High night-time temperatures can also mean there is little chance for the body to cool down, it added.
Meanwhile, Defra said the possibility of smog meant people should cut out unnecessary car journeys and advised asthma sufferers to use their inhalers.
"Sunny, warm weather over southern and eastern England over the next few days is likely to lead to the first summer smog episode of 2006," it said.
The high ozone levels were forecast from Saturday, and were expected "to persist until at least Monday", Defra said.
Ground level ozone is formed when sunlight acts on nitrogen dioxide and other atmospheric substances. Pollutants that cause smog come from different sources, including petrol and other fuels.
Government advice states that most people will experience no ill effects, but those suffering from lung complaints such as asthma, and particularly the elderly, may see their symptoms worsen.
People who have noticed that their breathing is affected in hot weather are advised to avoid "strenuous outdoor activity", particularly in the afternoon.
Children with asthma should be able to take part in games, but may need to increase their use of reliever medicines beforehand, the advice says.