ULEZ: Why are councils taking London's mayor to court?

Users of non-compliant vehicles must pay a £12.50 daily charge to drive within the zone

Five Conservative councils are at the High Court on Tuesday trying to block the expansion of the Ultra Low Emission Zone (ULEZ).

The London authorities - Bexley, Bromley, Harrow, Hillingdon - and Surrey County Council are seeking a judicial review of London Mayor Sadiq Khan's decision to extend the ULEZ to outer London.

From 29 August, vehicles driving anywhere in the capital which don't comply with emissions standards will be liable to a £12.50 daily charge.

• ULEZ expansion: Contested claims examined

• How ULEZ generated £224m in one year

• Impact of mayor’s ULEZ expansion questioned

What are the grounds for the legal challenge?

There are three.

1. Exceeded powers

The ULEZ was introduced in central London in 2019 and extended to within the North and South Circular roads in 2021.

The councils argue this expansion - covering an area 16 times as big - is of a different magnitude.

They say it warranted being treated as a new scheme - not a variation - imposing on the mayor more legal and consultative requirements.

2. Flawed consultation

The councils say there was an unfair and unlawful failure to disclose important information during the 10-week consultation process last year, thus affecting responses.

Data on how many vehicles complied with emissions standards was confusing and potentially misleading.

There are questions around data from Automatic Number Plate Recognition (ANPR) cameras and whether it reflected the number of vehicle owners likely to be affected.

3. Arguments about scrappage

These relate to the £110m fund which dispenses money to some owners of non-compliant vehicles prepared to scrap them.

The councils argue that people living in London were not given enough details to enable them to make an informed response to the consultation.

They weren't told who would be eligible - largely only those on benefits - or how much was on offer.

It is also claimed that there was an unlawful failure to consult people living outside London and bordering the zone who are not eligible for money from the fund.

What is the mayor's defence?

He has a statutory responsibility to take measures to improve London's air quality. Expanding the ULEZ is a variation of the existing scheme introduced lawfully under powers in the Greater London Authority Act.

Compliance information was set out clearly in the consultation process and was sufficient to enable an intelligent response.

Predictions of future compliance were based on established modelling methods and assumptions. ANPR camera data provides a good basis for these estimates.

It was obvious from previous scrappage schemes that this one would be substantial, confined to Londoners and targeted at eligibility criteria like low income and disability.

How much is this about air quality?

The mayor says the expanded ULEZ will cut pollution and improve health.

The extent of this is disputed.

However it is unlikely the debate around this will feature much in this hearing.

The case is focused on whether the mayor conducted a fair and lawful process, with this specific scheme, one which adequately accounted for the interests of the people it will affect.

What are the councils' chances?

The mayor's agency Transport for London (TfL) is expert at drawing up big schemes - and presumably considerable effort will have gone into ensuring this is within its powers and legally sound.

Allowing this to come to a substantive hearing means a judge considered the councils have a realistic chance of success.

Yet the hurdle is high and the odds will be with the mayor.

A majority of cars driven in London are ULEZ compliant

What's at stake?

The mayor has made air quality one of his core policy areas - he says there's no time to deny, dither or delay.

But this was not in his 2021 manifesto. It has become a potent political issue.

• Illegal levels of air toxins found in many boroughs

• ULEZ expansion: How many Londoners are affected?

• Labour MPs call for ULEZ scrappage scheme review

One of the reasons for trying to get this done by the summer is to allow as much time to elapse as possible by next year's mayoral election when he is seeking a third term.

Will feelings by then have calmed?

A decision is likely to come towards the end of July, only a month from the planned start date.

If any part of the councils' claim is successful - and the mayor is found to have acted unlawfully to some degree - there could be a delay while "remedies" are found or parts of the process repeated.

This could only add to the political difficulties Sadiq Khan is facing over ULEZ.

Follow BBC London on Facebook, Twitter and Instagram. Send your story ideas to [e-mail]