Air pollution is forcing the UK government back to court for the seventh time.
The environmental legal group Client Earth says minsters failed to conduct their recent public consultation on clean air properly.
It also says the government’s pollution plans for Scotland, Wales and Northern Ireland are far too vague.
A government spokesman said ministers were committed to improving air quality.
Client Earth’s case centres on the disparity between the consultation document the government produced for the public, and a related technical document from government experts.
The technical document makes it clear that the best way of combating pollution is for councils to set up clean air zones and then to charge vehicles for entering them.
But the consultation document – produced around election time following a court ruling – said councils should try all other measures before imposing charging zones.
Critics said the consultation had been rendered toothless by ministers’ wish to avoid political controversy.
Client Earth says the difference between the two documents could open the government to judicial review from members of the public or councils who feel they were not properly consulted.
Its spokesman Tim Reid told BBC News: “The government needs to consult on this issue – it’s just not behaving properly. We are dismayed to be taking them back to court again.”
The group thinks that even a brief consultation of one or two weeks on charging zones would insulate ministers from further legal challenge as they prepare their air pollution strategy by the 31 July deadline imposed by a previous court ruling.
The other issue before the courts is the relationship between the UK and the nations.
Air pollution is a devolved issue, but Client Earth argues that the UK government must take overall responsibility for EU laws. It says the pollution strategy for Wales, Scotland and Northern Ireland is “plans about plans”.
A government spokesman said: “We have jointly consulted with the devolved authorities on proposals to further improve the nations’ air quality and will publish a final air quality plan by 31 July.
“The draft plan sets out steps the UK, Scottish government, and Welsh governments and the Department of Agriculture, Environment and Rural Affairs in Northern Ireland propose to take to improve air quality in our towns and cities.”