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Sadiq Khan has triggered an emergency air quality alert as soaring temperatures are expected to bring dangerously toxic air to the capital.

The Mayor of London’s alert will see warnings displayed on the underground, on road signs and at bus stops as extremely hot conditions means high pollution levels are forecast.

It comes as King’s College London air quality researchers renewed warnings that toxic air will sweep across the continent with southerly winds.

Highs of 34C are forecast in west London, which would make it the hottest June day since the 35.6C recorded in Southampton on June 28 1976.

Mr Khan’s warning aims to ensure Londoner’s are aware and able to take “extra precautions” when travelling on the capital’s streets in the hot weather on Wednesday.

And experts say the worst air quality in the capital is likely to come on Wednesday afternoon, as temperatures soar.

And in statement, City Hall advises commuters to alter their normal routes if they are expecting to travel through London’s most polluted areas.

A spokeswoman for the Mayor’s office said: “Due to the continued warm and sunny weather conditions high levels of pollution are forecast, particularly near busy roads.

“As a result of this the Mayor has triggered a London-wide air quality alert on the transport system to ensure Londoners are fully aware and able to take any extra precautions needed.

“It is important that Londoners who are vulnerable, such as asthma sufferers, and children and elderly people, are able to take appropriate measures to protect themselves.

“These might include carrying inhalers or modifying journeys into work to avoid the most polluted routes.”

Pollution in London 6 show all Pollution in London 1/6 This image taken near the Royal Observatory in Greenwich shows a thick layer of smog hanging over London Anna Rolls 2/6 This image, taken in December 2016, shows the smog over London Pete Buckney 3/6 This shot taken in 2015 from Hampstead Heath shows smog hanging over the centre of London Getty Images 4/6 Pollution: thick smog hangs over London's Docklands PA 5/6 Pollution hanging over the City of London 2011 FILE IMAGE AFP via Getty Images 6/6 Smog: A thick layer of smog visible above London's skyline William Smith/@williamsmithorg 1/6 This image taken near the Royal Observatory in Greenwich shows a thick layer of smog hanging over London Anna Rolls 2/6 This image, taken in December 2016, shows the smog over London Pete Buckney 3/6 This shot taken in 2015 from Hampstead Heath shows smog hanging over the centre of London Getty Images 4/6 Pollution: thick smog hangs over London's Docklands PA 5/6 Pollution hanging over the City of London 2011 FILE IMAGE AFP via Getty Images 6/6 Smog: A thick layer of smog visible above London's skyline William Smith/@williamsmithorg

Earlier this year, Mr Khan took unprecedented steps to use “black” or “very high” air quality warnings in parts of the capital as pollution soared.

Experts said the 10/10 alert meant “adults and children with lung problems, adults with heart problems, and older people, should avoid strenuous physical activity.”

The warning comes as Mr Khan looks to roll out extra charges on the “dirtiest” vehicles using roads in London.

In its statement published on Wednesday, City Hall added: “Given that much of the toxic pollution today in London is being blown over from the continent, we must continue to work closely with our neighbours to reduce emissions across Europe, even after Brexit.”