Boris Johnson could be forced by the government to rethink his decision to scrap the western extension to the Congestion Charge zone in a row over air quality.
Following a public consultation the mayor announced last year that he would be scrapping the controversial scheme.
He also suspended the third phase of the Low Emission Zone, saying it could hurt businesses in the capital.
But now it has emerged that the government could force the mayor to either reconsider the move or come up with alternative schemes to offset any detrimental effects to air quality.
The Department for the Environment, Food and Rural Affairs is concerned that if strict European directives on pollution are not met the taxpayer could be fined up to £300m.
Defra can use its powers, as laid out in the Greater London Authority Act 1999, to force the mayor to come up with alternative schemes to improve air quality.
If enacted, it would be the first time government has used these powers to challenge the London mayor and means that ministers could force Mr Johnson to reverse policies if they clash with national decision-making.