Plans to build a third runway at Heathrow may breach the Government's own climate change legislation if other sectors do not make big cuts to emissions, an independent advisory body has warned.
The Committee on Climate Change (CCC) said it had "concerns" over how the Department for Transport (DfT) had presented its case for expanding the hub in relation to greenhouse gas emissions.
The plans have come under fire over the potential to hit air quality for millions of people living near Britain's busiest airport, and the CCC warned the proposals could also have a much wider impact on industry.
The Government announced it had chosen to expand Heathrow over other schemes in October following a review by the Airports Commission.
In a letter to Business Secretary Greg Clark, the chairman of the CCC, Lord Debden, said: "The Committee has concerns about how it (DfT) presented the implications for greenhouse gas emissions from aviation in that business case."
The CCC has advised the Government that, in order to stay within environment laws, emissions from air travel in 2050 should not exceed 2005 levels.
The committee said it expects advances in technology to help to reduce emissions while air travel grows. There has been an 11% increase in demand in the last decade, although the latest data showed emissions are currently lower than in 2005.
However the Government's plan for Heathrow estimates that emissions from aviation will rise by 15% as demand surges by 60% by 2050.
"The committee is now concerned that there is scope for misunderstanding of the DfT's position based on the business case for a third runway at Heathrow," Lord Debden wrote.
"Using the Government's publications, it is not possible to assess whether the investment makes sense when emissions conform to the planning assumption."
Lord Debden warned that plans to expand Heathrow while keeping current targets meant "all sectors would have to prepare for correspondingly higher emission reductions by 2050".
"My committee has limited confidence about the options for other sectors to go beyond these levels by 2050," he wrote.
The Department for Business, Energy and Industrial Strategy told BBC News the Government agreed with the Airports Commission's assessment that a new runway at Heathrow will not breach emissions targets.
A spokesman said: "We are considering how we will continue to reduce our emissions across the economy through the 2020s and will set this out in our emissions reduction plan, which will send an important signal to the markets, businesses and investors.
"Our commitment to meeting our Climate Change Act target of an at least 80% emissions reduction below 1990 levels by 2050 is as strong as ever."