The London mayor has formally opposed plans for a third runway at Heathrow Airport, arguing the expansion threatens environmental targets and transport networks.
City Hall and Transport for London (TfL) submitted a joint response to the government's draft Heathrow Expansion National Policy Statement (HENPS) consultation.
Sir Sadiq Khan claimed the strategic case relied on outdated evidence that "fails to reflect current climate laws and travel habits".
The government said it was reviewing the response, while Heathrow questioned many of the claims made in City Hall's submission.
The airport said the expansion was the "most important economic growth opportunity available to the UK".
But Sir Sadiq said he saw no evidence the proposals were compatible with a credible path to net zero.
"The evidence suggests Heathrow expansion is a risk that we simply cannot afford to take."
City Hall said additional flights could reverse the progress in reducing air pollution in communities surrounding Heathrow brought by the Ultra Low Emission Zone (Ulez).
The submission raised several issues regarding infrastructure and finance:
TfL estimates targets in the draft policy could generate 60,000 additional daily highway trips, adding 22 million road journeys annually to the M4 and M25
Capacity upgrades would be needed for the Elizabeth and Piccadilly lines, alongside new southern and western rail links. TfL put the potential surface access cost at £9bn, noting no funding agreement exists
TfL said a 58% increase in flights would reduce predictable noise respite periods for local residents from 50% to 25% of the day
"The whole case for Heathrow expansion relies on outdated evidence that fails to take into account the urgent challenges we face today," Sir Sadiq said.
"I will continue to vehemently oppose these plans as I seek to build a fairer, safer, greener, more prosperous London for everyone."
Heathrow Airport Limited is preparing plans for a new 2.1 mile (3.5km) northwest runway - which would require realigning a section of the M25 - with a final decision expected following a parliamentary vote in 2029.
A spokesperson for the airport rejected claims that expansion was incompatible with net zero goals - and said the airport was committed to a completely privately-funded noise insulation scheme for local homes.
On transport infrastructure, Heathrow stated its expansion plans include widening a busy stretch of the M25, adding that it will present "viable and funded solutions" for required public transport upgrades as part of the planning process.
"This is about much more than a new runway.
"It's about unlocking growth and over 100,000 jobs for the whole country, boosting British trade by £150bn, making travel more affordable and revitalising the UK's steel industry."
The mayor's stance is backed by a coalition of London and home counties council leaders - including Wandsworth, Richmond, Kingston, Hillingdon, Southwark, Sutton, and Windsor and Maidenhead.
In a joint letter to Transport Secretary Heidi Alexander following the consultation's close, the cross-party group called on the government to withdraw support for the third runway.
However, the Back Heathrow campaign pushed back against the councils, claiming their opposition was "not representative" of local residents.
Executive director Parmjit Dhanda said more than 100,000 local members support the expansion, citing the potential for jobs, trade, and economic growth, adding that a new runway was "way overdue".
The Department for Transport, which threw its support behind expansion under Keir Starmer's premiership in 2025, said ministers would "carefully consider all responses before setting out next steps later this year".
"We have always been clear that expansion will only go ahead if it is delivered in line with our legally binding climate change obligations, and meets strict environmental rules on air quality and noise pollution," a spokesperson for the department added.
The position of the mayor and TfL against expansion at Heathrow hasn't changed -they have always had concerns over noise pollution, public transport and extra traffic.
What is different this time is they think the modelling is outdated, and the language seems to have toughened.
Perhaps Ulez has empowered the mayor to be more direct?
TfL said since the pandemic, the world had changed with a decline in business travel. They also said the figures being used to back expansion are over ten years old and no longer relevant.
It leaves the mayor potentially on a collision course with the pro-expansion lobby - including those within his own party.
Additional reporting by Charlotte Lillywhite, Local Democracy Reporting service.