Transport Secretary Chris Grayling admitted the government backed a third runway at Heathrow without fully understanding the implications of ground-breaking new evidence on vehicle emission standards.
Ministers insist the west London airport can expand within EU limits on air pollution, which are currently being widely breached in the capital.
But a study for the government, supporting its third runway decision, was not based on the latest international analysis by experts, which showed emissions from some diesel vehicles are worse than previously claimed.
“Further work is needed to understand the implications of this evidence,” Grayling told MPs in a letter ahead of his appearance in front of the Commons environmental audit committee this week. “But our initial assessment suggests that revised forecasts would be likely to be within the range of scenarios already considered by our re-analysis (on air quality).”
However, committee chairwoman Mary Creagh said: “We will want to hear from the minister how the government can meet air quality standards given what we now know about real-world emissions, which are higher than used in the government’s business case (for a third runway).
“We are also concerned that the plans for low-emission vehicle uptake and improvements in public transport are over-ambitious.”
Lord Deben, chairman of the committee on climate change, also raised doubts over whether another runway could be built without breaching Britain’s legally binding carbon targets.
The Tory peer said these could be met if levels of aviation emissions were the same as in 2005 but the department for transport had “left open” for them to rise.
“What is not possible is to have a business plan that suggests that, overall, aviation would increase the emissions by 15%,” he added.
Other sectors such as the steel industry would find it “practically impossible” to make sufficient extra cuts to offset such a large rise. 
Ministers insist the government’s 2015 Air Quality Plan and new measures around Heathrow will ensure London will not be breaking nitrogen dioxide levels in the mid 2020s when the new runway is due to open.
But the government was defeated earlier this month for the second time in the courts over this blueprint.
The ruling in the Judicial Review, brought by environmental lawyers ClientEarth, called the plan “woefully inadequate”.
“We are carefully considering what this means for the airport capacity programme,” Grayling added in his letter.
The Cabinet minister emphasised that final consent for another runway would only be given if the government believed it would not breach the UK’s compliance with pollution limits.
A government spokeswoman said: “Improving air quality is a priority and we are determined to cut harmful emissions. Our plans have always followed the best available evidence – we have always been clear that we are ready to update them if necessary.”

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