People with asthma and other respiratory problems are being warned to stay indoors and avoid exercise today with thunderstorms and air pollution set to cause high smog levels that could affect their health.

Health experts said up to 15% of the population could be at risk after the government issued its first smog warning since the heatwave of two years ago, when it is estimated up to 800 premature deaths occurred as a result of poor air quality.

The Met Office has forecast high ozone levels for today and tomorrow in London, East Anglia and south east England.

Professor Frank Kelly, director of King's College London's environmental research group, said the typical symptoms those at risk would feel were, "a tightening of the chest, or even some pain in the chest".

Professor Martyn Partridge, of the National Heart and Lung Institute, said: "Over 80% of people with asthma find air pollution makes their asthma symptoms worse."

Ben Bradshaw, minister for local environmental quality, urged those with heart and lung disease to take, "sensible precautions, such as avoiding exertion outdoors on hot afternoons".

A spokeswoman for the charity Asthma UK advised those at risk to be particularly careful during thunderstorms, when air quality is poorest.

Seven smog alerts were issued during the 2003 heatwave, while there were none last year.