A campaign group has launched a legal challenge over the expansion of Gatwick Airport.
Communities Against Gatwick Noise Emissions (CAGNE) is arguing that the government's decision to approve the construction of a second operating runway at the West Sussex airport was flawed.
The Department for Transport said it was unable to comment due to ongoing legal proceedings.
A spokesperson for London Gatwick said they were aware of the prospective judicial claim against the Northern Runway Project.
"Dependant on the next steps, we will determine our position and the extent of our involvement in legal proceedings," they added.
CAGNE said the project's impact on greenhouse gas emissions and climate change was not properly assessed.
The campaign group is also claiming the planning process failed to properly factor in additional noise pollution and impact on air quality.
They are urging Transport Secretary Heidi Alexander to overturn her decision.
Chancellor Rachel Reeves has previously said the expansion would create "thousands of jobs and billions in investment".
"This extra runway at Gatwick will mean that people going on holiday will have a greater choice of destinations, it will mean lower costs for a family holiday," she added.
A government source also described the plans as a "no-brainer" for economic growth.
Plans for the second runway at London Gatwick - already Europe's busiest single-runway airport - were submitted for consideration in July 2023.
The £2.2bn privately-financed project involves moving its current backup runway to bring it into use alongside other developments, such as making the terminals bigger.
It is estimated the expansion will result in more than 100,000 extra flights per year by the late 2030s, the airport says.
Passenger numbers could rise to up to 80 million, it added.
The government approved the development in 2025, following a year-long examination period.
Sally Pavey, chair of CAGNE, said increased low-cost flights would export sterling from the UK economy and place an "unsustainable burden on the inadequate infrastructure".
"The long-term effects of increased emissions on future generations and the planet cannot be addressed with aviation profits," she added.
"This decision to allow Gatwick's expansion makes no economic or ethical sense, so we must challenge it through the legal process."
Gatwick's is the latest in a string of UK airport expansion approvals, most recently Luton's in April.
The government has said it backs a third runway at the country's biggest airport, Heathrow.