Final approval for a third runway at Heathrow should not be granted until the airport demonstrates it can meet key environmental conditions on climate change, air quality and noise, a committee of MPs has said.
The Davies Commission recommended the construction of the controversial third runway in a report in July, but the Government has not yet made clear whether it will give the scheme the go-ahead. Ministers have promised to announce their decision by the end of this year.
In a new report, the House of Commons Environmental Audit Committee warned that a failure to deal with environmental concerns could lay the scheme open to legal challenge.
The cross-party committee called on Transport Secretary Patrick McLoughlin to offer assurances that all environmental conditions will be met before Parliament's approval is sought for the expansion of the west London airport.
Committee chairman Huw Irranca-Davies said: "To defer dealing with the environmental impact of a third runway would be irresponsible and could lead to legal challenges as a result of the potential damage to public health from increased air pollution and noise.
"If the Government decides to accept the Commission's recommendation for a third runway in principle, we will seek assurances from the Secretary of State for Transport that environmental conditions will be met before it is given final approval."
The MPs said that the airport must demonstrate that it can reconcile Heathrow expansion with legal air pollution limits, as well as committing to a night-flight ban and to covering the cost of improved transport links and showing that an expanded airport will be less noisy than one with two runways.
"The Government has a duty to reduce illegal levels of air pollution in London to protect the health and well-being of its population," said Mr Irranca-Davies.
"The communities living near to the roads around Heathrow already put up with noise and extra traffic, it would be quite unacceptable to subject them to a potentially significant deterioration in air quality as well. Increased pollution should certainly not be permitted on the grounds that other areas of London are even more polluted."
The Government should put in place a strategy to deliver carbon emissions from aviation no higher than 2005 levels by 2050, in line with the economy-wide target set by the Climate Change Act, said the report
"Even without expansion, aviation is on track to exceed its climate change target. We heard evidence that those targets might be met in theory, but at present there is a policy vacuum and evidence-based scepticism as to whether they can be met in practice," said Mr Irranca-Davies.