Sadiq Khan was today under pressure to explain why he is expanding London's ultra-low emission zone as it emerged he once admitted it was not the best way to improve air quality.

In written correspondence from July 2021, unearthed by the Daily Mail, the London mayor said he had 'no plans' to extend the Ulez to cover Greater London.

He added that air quality on these roads 'will be more effectively addressed through targeted, local measures'.

MPs last night said the disclosure was proof the expansion, which comes into force next week, is nothing more than a cynical 'cash grab'.

It also emerged that emissions-based schemes in cities across England have netted more than £400million for town hall bosses since 2021, with London's Ulez accounting for more than £300million of this.

In written correspondence from July 2021, unearthed by the Daily Mail, Sadiq Khan said he had 'no plans' to extend the Ulez to cover Greater London (File image)

It came as ministers faced calls to pass laws so they can quash hated traffic schemes such as Ulez.

No 10 has been told not to block the Ulez expansion by government lawyers who warned the move would be rejected by the courts if challenged.

Cabinet ministers had been mulling a challenge based on a little-known legal power allowing them to reject a London mayor's transport strategy if it is 'inconsistent with national policies'.

But they were told that any such move would likely fail if challenged in court as the Ulez scheme is apparently not 'inconsistent' with their own air pollution improvement aims.

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An emissions scheme initially known as the Low Emission Zone (Lez) was first launched in London in 2008.

But the zone only covered Greater London.

In April 2019 the Ulez scheme was introduced in Central London, which tightened restrictions further to cover older vehicles.

And in October 2021 it was expanded to cover all of inner London up to the capital's North and South Circular roads.

At the time there were fears Mr Khan was plotting to expand the scheme to cover the whole of outer London and in a written question in July 2021 he was asked: 'Do you have any plans to extend the Ulez to the outer London boundary, and if so, when would this be likely to happen?'

Mr Khan replied: 'I have no plans to extend the Ulez to outer London... Combined with the Ulez expansion in October, up to but not including the North and South Circular, this will ensure that 92 per cent of roads London-wide will comply with legal limits for NO2 [nitrogen dioxide] by the end of 2021.

'Air quality on the remaining roads will be more effectively addressed through targeted, local measures.

'I believe that this strikes the right balance to deliver the biggest air quality improvements as quickly as possible.'

But despite this, Mr Khan is pressing ahead with widening the Ulez to cover all of outer London next Tuesday.

It means hundreds of thousands more cars will pay a £12.50 daily charge if they drive older polluting vehicles which don't meet the tough emissions standards.

It will also clobber motorists in the home counties close to the outer London border who need to cross it for work and leisure purposes.

Steve Tuckwell, the Tory MP for Uxbridge and South Ruislip, whose west London seat will be affected by the levy rollout, said: 'We've known all along that Ulez expansion was all about the money for Mayor Khan.

'He needs to come clean and admit his Ulez expansion is nothing more than a cash grab.'

Fellow Tory Louie French, whose Old Bexley and Sidcup constituency will also be impacted by the expansion, said: 'Most Londoners can see very clearly that drivers in Greater London are being used as a cash cow to fill the black hole in TfL's finances caused by seven years of Sadiq Khan.

'Sadiq Khan's own independent impact report highlighted that Ulez expansion to the suburbs would only have a negligible impact on improving air quality.'

Steve Tuckwell, the Tory MP for Uxbridge and South Ruislip, whose west London seat will be affected by the levy rollout, said: 'We've known all along that Ulez expansion was all about the money for Mayor Khan (File image)

Ministers are unable to block the expansion because of powers devolved to Mr Khan in legislation introduced under Sir Tony Blair's Labour government.

But they are facing calls to re-write this legislation so schemes like Ulez can be blocked by ministers in future and even retrospectively.

Theresa Villiers, the Tory MP for Chipping Barnet and a former environment secretary, said: 'The only guaranteed way to reverse Ulez expansion is to elect a Conservative mayor next May [in the mayoral election].

'But that's next May and we face several months of this scheme operating.

'The other way is to legislate to give ministers the power to stop it.

'Ministers could introduce their own primary legislation and I would certainly urge them to do so as soon as possible. This is something I've raised with them and so far they've declined to do so.'

Prominent road laws lawyer Nick Freeman, also known as Mr Loophole, said he agreed with the government lawyers' assessment.

But he added: 'Ministers need to stop talking the talk and start walking the walk and bring in new laws so they can rein in Mr Khan.

'It's that simple. Then we'll see if they really do intend to do anything about it.'

A spokesman for Mr Khan said: 'The Mayor has been clear that the decision to expand the Ultra Low Emission Zone London-wide was not an easy one, but necessary to tackle toxic air pollution.

'Evidence is consistently published highlighting further terrible impacts of exposure to air pollution.

'The Mayor recognises we always need to evolve and adapt our policies to make sure we address the current challenges.'