The admission puts the airport expansion in jeopardy because the government has said planning consent will not be granted if it cannot be delivered within current air quality and climate change laws.
The independent impact assessment commissioned by the government warns that building a third runway could delay compliance with air quality laws. It states that there is "a risk that the option will delay compliance" with air pollution limits.
The assessment, carried out by an external consultancy for the Department for Transport, goes on claiming that compliance with the law won't be a problem after 2030. Yet the document also reveals that this prediction is based on the assumption that real-world car emissions will be improved significantly over the next few years - a claim which is speculative.
The government also appears to contradict the conclusion of the independent analysis in its policy document, where it claims that "with a suitable package of policy and mitigation measures, including the Government's modified air quality plan, the Heathrow Northwest Runway scheme would be capable of being delivered without impacting the UK's compliance with air quality limit values".
Commenting on the announcement, Greenpeace UK executive director John Sauven said:
"Now we have it straight from the horse's mouth. The government-commissioned assessment shows a third runway risks delaying action to bring air pollution down to legal levels for years. And even after 2030, ministers' hopes of complying with air quality laws are based on drivers using less polluting cars. But with an air pollution crisis already linked to over 9,000 premature deaths a year in London alone, people's health cannot be dependent on rose-tinted assumptions and a notoriously unreliable car industry. It's time for ministers to come clean. They don't have a solution to the extra emissions from a third runway that will cause more air pollution and climate change. Going ahead with this project would be reckless and unlawful."