The Ultra Low Emissions Zone (ULEZ) - which could also now come into force earlier than planned - will require drivers of the 2.5m oldest and dirtiest vehicles to pay a charge. It is intended to act as an incentive to drivers to use cleaner vehicles or alternative transport to reduce the levels of nitrogen dioxide, a toxic gas produced by diesel vehicles. The value of the charge has yet to be decided.

Under Khan’s plans, which will now be subject to a public consultation, the ULEZ will stretch from the north to south circular roads in London rather than just the much smaller congestion charge zone in central London as currently planned. Officials said the area covered will more than double in size.

Khan said Johnson had been too slow to act and the government had been “hopelessly inactive” on the issue. Officials said the ULEZ, under a consultation to be published within a week, could now come into force as soon as 2019 rather than the original plan of 2020.

“I have been elected with a clear mandate to clean up London’s air - our biggest environmental challenge,” Khan said at a school in east London. He said London had only acted on pollution in the past after emergencies, such as the Great Smogs of the1950s: “But I want to act before an emergency, which is why we need big, bold and sometimes difficult policies if London is to match the scale of the challenge.”

The mayor’s office also said an extra charge on the most polluting vehicles would be brought in from 2017, which would be administered by the congestion charge system but be separate to the congestion charge. It is not yet clear what that charge will be, or how many vehicles will be affected by the ULEZ.

Simon Alcock of ClientEarth which has sued the government over the illegal pollution levels in London and other cites said the mayor’s plan was a “hugely positive announcement”.

“We will have to wait and see if the detail of the mayor’s proposals matches his ambition. With air pollution causing over 9,000 deaths a year in London it is vital that all options to solve this problem are on the table. It will be crucial that the ULEZ ensures vehicles meet the most stringent emission standards when driving on London’s roads, not just in discredited laboratory tests,” he said.

“Today’s announcement, coming so early in the new mayor’s term, should send a clear message to the UK government that ambitious and bold action is needed. The government must now up its game so that the whole country can breathe cleaner air.”

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