Mayor of London
Sadiq Khan has issued a toxic air alert. Yui Mok/Getty Images

A three-day toxic air alert has been issued for London, with residents in the UK capital warned by Mayor Sadiq Khan about the 'public health emergency'.

Pollution may rise to moderate levels in 17 boroughs in the city, with two areas likely to experience 'high' levels of pollution on 19 January, City Hall said as it issued the warning.

However, the two areas where pollution could be high —Westminster and the Square Mile — should return to moderate levels of pollution by the end of the week.

Khan told the Evening Standard: "London's dirty air is a public health emergency.

"We will continue to use all the technology at our disposal to inform Londoners about levels of air pollution in their neighbourhoods."

The Department for the Environment's advice for areas hit by moderate air pollution states: "Adults and children with lung problems, and adults with heart problems, who experience symptoms, should consider reducing strenuous physical activity, particularly outdoors."

However, in areas where pollution may reach high levels, it added: "Adults and children with lung problems, and adults with heart problems, should reduce strenuous physical exertion, particularly outdoors, and particularly if they experience symptoms.

"People with asthma may find they need to use their reliever inhaler more often. Older people should also reduce physical exertion."