The smog was visible as a haze over the London skyline on Friday morning, the start of a four-day weekend when many people were expected to head outdoors to enjoy the continuing warm weather.

Britain has basked in sunshine for weeks and the Department for Environment and Rural Affairs said warm and still conditions over the next few days would likely cause a "high pollution episode" in England and Wales.

It urged the public to take "sensible precautions", saying: "Avoiding exercise outdoors in the afternoon can reduce exposure to ozone, and avoiding making unnecessary short car journeys wherever possible can reduce the formation of ozone".

Ground level ozone is formed when sunlight acts on nitrogen dioxide and other atmospheric substances close to the ground.

During high pollution episodes, levels of nitrogen dioxide, ozone and polluting particles are raised and can exacerbate lung diseases including asthma. But the ministry said most people should not suffer ill effects.

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