Illegal levels of toxic air pollution have been recorded in outer London ahead of plans to expand the Ultra-Low Emission Zone (ULEZ) next year.

The mayor of London and TfL are currently holding a public consultation on plans to extend the ULEZ to cover the whole of Greater London by August 2023 in a bid to tackle poor air and congestion.

It follows an initial expansion of the zone in October last year to cover the areas up to, but not including, the north and south circular roads, taking an estimated 50,000 older, more polluting vehicles off the road each day.

Drivers of cars that do not meet rigorous minimum emissions standards must pay a daily charge of £12.50 to enter the zone.

Some motorists in outer London have spoken out against plans to expand the zone to cover all 32 boroughs, but clean air activists have said that ULEZ “must be expanded without delay” after illegal levels of Nitrogen Dioxide were found in some areas of outer London.

According to the latest figures, 15 air quality monitors across London recorded levels of NO2 above legal limits, while not a single air quality monitor recorded levels that met recommended limits set by the World Health Organisation.

Legal limits for NO2 were supposed to be met in the UK by 2010 at the latest.

Illegal levels of NO2 were recorded in Kingston-upon-Thames, Brent and Sutton, none of which are currently covered by the ULEZ, but could be if Sadiq Khan presses ahead with plans to expand it further next year.

Oliver Lord, UK head of the Clean Cities Campaign, said, “if this isn’t a wake-up call, I don’t know what is”.

He added: “These limits were meant to be met 12 years ago, and when that eventually happens, they are still four times World Health Organization guidelines. We need greater certainty that London is on track to phasing out toxic diesel fumes. The Ultra-Low Emission Zone must be expanded without delay and the mayor’s plans for a central London zero emission zone need to be expedited. There is no time to waste.”

The plans have come under fire from the City Hall Conservatives, who have said that the mayor of London is “steaming ahead with plans he did not include in his manifesto”.

The group’s transport spokesperson Nicholas Rogers, who represents South West London, has previously said that Sadiq Khan has “a fundamental misunderstanding of the specific demographics of London, where outer boroughs have an average vehicle ownership of nearly 70 per cent”.

Sadiq Khan has said the public consultation over the plans is a “genuine consultation”, and that he will not press ahead with the proposal as it is if Londoners overwhelmingly reject it.

Last month, Barnet Conservative group leader, Dan Thomas, criticised the ULEZ expansion saying: “We can always make improvements, but we don’t need drastic measures that will cost more to drive to work.”

Londoners have until July 29 to respond to the consultation, which can be found on the TfL website.