The decision on whether to approve a new road tunnel under the Thames has been delayed by a month.
Transport minister Paul Maynard said the delay would allow the department to consider the project's environmental impacts - writes bbc.com.
The Green Party said the "toxic tunnel" could damage Sadiq Khan's plans for improving the quality of London's air.
However, a spokesman for the mayor said the Silvertown Tunnel would provide significant benefits for London.
In his ministerial statement, parliamentary under secretary of state Paul Maynard announced the deadline would be extended to 10 November.
"This extension is to enable further consideration of the recent responses to the secretary of state consultations on the scheme which relate to the updated UK plan for tackling roadside nitrogen dioxide concentrations published by government on July 26, 2017," he said.
"The decision to set a new deadline is without prejudice to the decision on whether to give development consent."
Should the tunnel be approved Transport for London (TfL) said the tunnel would be expected to open in 2022 or 2023.
A spokesperson for the Mayor of London said: "TfL and the mayor are also determined to ensure the Silvertown Tunnel doesn't have a detrimental impact on our environment.
"That's why the plans have such a focus on cleaner public transport, with only buses with the highest emission standards using the tunnel and significant investment in pedestrian and cycling improvements in the area."
Caroline Russell, Green Party member of the London Assembly, said: "The mayor's toxic tunnel would blow the UK's cleaner air plans out of the water in east London, people there already suffer from living in one of the most polluted parts of London."
Read more news of London on our city site.