A third option would see both the Ulez expand and the “clean air charge” – which he said would cost a “small sum of money” but probably more than a bus ticket – being introduced.
A fourth option would see Mr Khan press ahead with already controversial plans to impose a Greater London boundary charge of about £3.50 a day on out-of-town motorists driving into the capital.
He wants to make the change before his second term of office ends in May 2024.
Transport for London has been asked to investigate the feasibility of the four options – and also the longer-term aim of introducing the world’s first road-user charging scheme in London by the end of the decade.
Mr Khan’s announcement on Tuesday came after an expert report commissioned by City Hall said car journeys needed to be reduced by 27 per cent by the end of the decade if London were to meet its ambitions on climate change.
Mr Khan said the clean air charge could cost more than a bus fare – currently £1.55 but due to increase in March as part of the delayed annual fares hike of about five per cent.
He said only drivers of hybrid or electric vehicles were likely to be exempt.
“I envisage [it] being small sum of money that would be used to nudge the behaviour of the vast majority who haven’t got the cleanest vehicles,” he told a City Hall press briefing.
“I suspect the scheme that TfL may come up with is anybody who has not got an electric vehicle or hybrid vehicle may have to pay this clean air charge.
“I would imagine the sort of thing TfL would be looking at is: what is the cost of a bus fare? What we are trying to do is nudge people away from petrol and diesel vehicles to either walk, cycle, use public transport or, if they are going to drive, use an electric vehicle.
“That is the sort of ballpark we are talking about, so it’s cheaper to jump on a bus than it is to jump in your car.”
Mr Khan said he wanted to have an “earnest conversation with Londoners” to secure backing for his aim to make London “net zero” in terms of carbon emissions by 2030.
He appears to have become bolder in pursuing his environmental aims after the successful expansion of the Ulez to the suburbs in October – 92 per cent of vehicles in the zone now comply with the emissions rules.
Another factor is his appointment as chairman of the C40 group of world cities committed to tackling global warming – a role that has seen him dubbed the “king” of green mayors.
“I have got to make sure there is a disincentive to drive your car, particularly if it is petrol or diesel, when there are alternatives, like public transport,” he said.
Mr Khan currently has the power to widen the £12.50-a-day Ulez, impose the clean air charge or do both – but not to impose the Greater London boundary charge.
Transport Secretary Grant Shapps has made clear his opposition to the boundary charge, which has been dubbed “Checkpoint Chigwell” by the mayor’s critics.
The Department for Transport declined to be drawn on Mr Khan’s plans, saying only that decisions on road user charging were devolved to the mayor and TfL.
Mr Khan, who has previously shunned encouragement from the London Assembly Greens to commission work on smart road-pricing, indicated he was now in favour of charging motorists according to the time of day travelled, distance travelled and level of congestion.
If introduced, this would replace both the Ulez and the £15 congestion charge, which is due to end at 6pm from next month.
Rosamund Adoo-Kissi-Debrah, whose daughter Ella’s death was found to have been caused in part by pollution from the South Circular Road, said: "We know measures like the Ulez work and have led to a 44 per cent reduction in roadside nitrogen dioxide concentrations in central London. We need more approaches like this, otherwise children like Ella will continue to die. On Monday 24th January Ella would have turned 18."
Adam Tyndall, of business group London First, said: “Reducing road journeys is an admirable aim but it must come with adequate public transport alternatives and not price vital trips off the road – whether that’s staff getting to early shifts in outer London or the army of delivery and service providers who keep the capital running.”
Zack Polanski, Green Party group spokesperson for the environment, said: “Today marks an historic win, showing that where green ideas lead, mayors eventually follow. This announcement is the first time the mayor has ever talked seriously about taking any action for a smarter system of charging for road travel.
“But the end of the decade will be too long to wait, and the technology necessary for a fair, smart charging scheme could in fact be in place much sooner. With the right political will and an honest public conversation about the details starting now, we can have this in place in less than three years.”
The RAC said less than a third of Londoners planned to switch to an electric car in the next five years.
Its head of roads policy Nicholas Lyes said: “At a time when the basic cost of living for Londoners is soaring, these proposals seem to be poorly timed, so we strongly urge the mayor to think again instead of defaulting to extracting more money from the pockets of London’s drivers.”
Caroline Pidgeon, chair of the London Assembly transport committee, said: “It is good to see the Mayor is focused on potential approaches to reducing congestion and car usage in the capital. But we’re running out of time to get this right and the sooner solutions are found the better.”
Mr Khan insisted the proposals, from work by Element Energy, were “completely separate” to efforts to win another Government bailout for TfL.
He said: “This is not about raising revenues to fill the black hole left by the pandemic in TfL’s coffers. If it was, we would be considering other options.”
He is seeking a 27 per cent reduction in kilometres travelled by car, compared to 2018 levels. Recent TfL figures have shown the number of road journeys have returned to levels almost as high as were seen pre-pandemic.
There would be exemptions and discounts for disabled Londoners and those on low incomes.
Research suggests that more than a third of car trips made by Londoners could be walked in under 25 minutes and two thirds could be cycled in under 20 minutes.
Mr Khan said he was targeting transport because vehicle emissions had fallen only seven per cent between 2000 and 2018, compared with a 57 per cent reduction in workplace greenhouse gas emissions and a 40 per cent reduction in emissions from homes.