A pollution warning has been issued for anyone with health problems to avoid any “strenuous activity” today after Britain was placed on health alert.

A potentially dangerous cloud is moving across the country, reportedly from Europe, which could cause fatal asthma attacks, experts have warned.

Risk to residents of Waltham Forest, Redbridge and Epping Forest is currently at a ‘moderate’ level.

The Department for Environment, Food and Rural Affairs (Defra), said air pollution was blowing in from Europe and mixing with home-grown smog to create the problem.

Officials warned this could trigger health difficulties for those with respiratory illnesses and the elderly.

Most of Waltham Forest is in the middle of the moderate scale, but it climbs higher around the Olympic Park in Stratford.

Levels are expected to drop tomorrow.

The number four moderate warning means anyone with health problems should consider what they are doing carefully.

In a statement, DEFRA said: “Adults and children with lung problems, and adults with heart problems, who experience symptoms, should consider reducing strenuous physical activity, particularly outdoors.”

Kay Boycott, chief executive of Asthma UK, said: "Two thirds of people with asthma find that air pollution makes their asthma worse, putting them at an increased risk of a potentially fatal asthma attack.

"When air pollution is high it's vital people with respiratory conditions including asthma check air pollution forecasts, carry their reliever inhaler with them at all times, and ensure that they are taking their preventer inhaler every day because this will help build resilience to asthma triggers like air pollution.”