The zone - which charges motorists £12.50 a day if their car does not comply with emissions standards - now covers the outer boroughs of the capital, having previously just applied in central London.

The city's mayor said extending the zone during the cost of living crisis had been "a difficult decision, but... a vital one", telling Sky News it was "an issue of social justice to clean up the air in our great city".

Mr Khan added: "The evidence is quite clear in relation to the consequences of air pollution. It does lead to, in London, around 4,000 premature deaths a year. It leads to children having stunted lungs forever, adults with a whole host of health issues from asthma to cancer, dementia to heart disease.

"We have the ULEZ in central London and it's managed to reduce the pollutants, the toxicity by almost 50%. A third fewer children admitted to hospital with air pollution problems because of the ULEZ in central London.

"But also the science shows that the 10 boroughs with the largest number of premature deaths is, guess where? In outer London. Look at those with illnesses in London linked with air pollution, more than two thirds live in, guess where? Outer London.

"They've not seen the benefits of ULEZ. After today, they will."

The expansion comes despite the London mayor facing stiff opposition to the plans from both the Conservatives and within the Labour Party - some of whom blamed the policy for the party's defeat in the Uxbridge and South Ruislip by-election last month.

Mr Khan's plans to extend the zone also faced a legal challenge by five local authorities - Hillingdon, Bexley, Bromley and Harrow in London, plus Surrey County Council - but it failed and the High Court ruled the expansion lawful.

It means that the zone will now be taken up to London's borders with Buckinghamshire, Essex, Hertfordshire, Kent, Berkshire and Surrey.

Those who drive in the newly expanded zone in a vehicle that does not meet minimum emissions standards will need to pay £12.50 a day fee or risk a £180 fine, reduced to £90 if paid within 14 days.

Transport Secretary Mark Harper attacked the roll-out, telling Sky News: "People can see it for what it is. It's a scheme to charge hard pressed motorists more money for making essential journeys and it will have almost no appreciable on air quality."

But Mr Khan defended his stance, saying: "I'm pragmatic. I'm not evangelical... I can see the evidence in relation to the consequences of air pollution, but also [the evidence that] ULEZ works.

"And actually I think Londoners want to see clean air in our city. They want to see their leaders taking bold action.

"What they don't want is politicians, for short term political gain, playing politics with public health and the climate emergency."

Read more:
ULEZ: Where is the new border?
Khan's office accused of 'cosy relationship' to 'silence' ULEZ criticism

To curb some of the opposition, the London mayor announced last month that the scrappage scheme will be extended to all Londoners through a £160m fund.

Applicants with a wheelchair-accessible car or van can get up to £5,000, while drivers of a standard car can receive up to £2,000 to scrap their vehicle. Motorcycle riders can also receive up to £1,000 for scrapping their bike.

Charities, traders and businesses can apply for larger grants to scrap, retrofit or replace a van or minibus.

Previously, grants to scrap a non-compliant car and replace it with a new one were only available to child benefit recipients, disabled people and those on low incomes.

City Hall says 90% of cars seen driving in outer London on an average day are already compliant.

It has also received more than 15,000 applications in the last week alone, while nearly £60m has already been committed in advance of the expansion to people, charities and businesses who want to scrap or retrofit their vehicles.

Following the political fallout of the Uxbridge by-election - in which the Tories kept hold of Boris Johnson's former seat by just 495 votes - a number of senior Labour politicians denounced the policy as responsible for the by-election defeat, with some, including shadow health secretary Wes Streeting, calling on Mr Khan to halt the expansion.

Sir Keir Starmer has also come under pressure to demand that Mr Khan delay extending the zone, with Mr Harper urging the Labour leader to make his position on ULEZ "clear".

The transport secretary told Sky News: "The government doesn't support [the expansion] but I don't have the legal power to block it. It's a decision by the Labour mayor, backed by the leader.

"The only person that can block it now is either if the mayor doesn't roll it out or the Labour leader threatens to de-select him if he goes ahead with it. And neither of those things have happened."

In response, a Labour source said: "The Conservatives are desperately hoping that the public forgets that clean air zones are their government's own policy, and that a Tory mayor created ULEZ.

"They've hammered motorists and stood idle while petrol prices soared, car insurance rocketed, and fewer potholes get fixed."