A toxic air alert has been issued for London after 'very high' pollution levels triggered a warning system, Mayor Sadiq Khan has announced.

Londoners were warned the high pollution level may require them to limit strenuous physical activity, even if they are very fit.

This is the first time the top-level alert has been declared under new measures to monitor air pollution in the capital.

Sadiq Khan calls air pollution in London a killer

"The shameful state of London's toxic air today has triggered a 'very high' air pollution alert under my new air quality warning system," wrote Mr Khan in a tweet.

He urged the capital's residents to use public transport if possible and asked drivers to turn their engines off if they had to stop for more than one minute.

The alert was issued after pollution detectors in the City, Westminster, north Kensington and three sites in Camden recorded very top levels of tiny pollution particles in the air this morning.

Airtext, which monitors London's air quality, defines 'very high' as the highest of 10 pollution bands. At this level, it advises most people to "reduce physical exertion, particularly outdoors, especially if you experience symptoms such as cough or sore throat".

Meanwhile the elderly, adults and children with lung problems, and adults with heart problems are advised to avoid strenuous physical activity altogether.

"People with asthma may find they need to use their reliever inhaler more often," says advice on Airtext's website.

Young pupils at Sir John Cass's primary school in Aldgate were kept indoors for most of the day due to pollution levels and freezing temperatures, reported the Evening Standard.

Last week, the mayor warned of a "public health emergency" as air pollution levels hit a "high peak" in central parts of the capital including Westminster and the Square Mile.

The new alert system, introduced last summer, shows pollution warnings on electronic signs at bus stops, tube stations and at the roadside, as well as issuing social media and text alerts.

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A City Hall spokesperson said the current episode of high air pollution had started on Saturday when 'high' levels were recorded and continued into Sunday, with the alert upgraded to 'very high' today.

Data from London's main monitoring system showed that the city broke the annual limit for air pollution just five days into 2017.