The government’s adviser on air quality has warned that ministers are “pulling the wool” over the public’s eyes to justify building a third runway at Heathrow.

Mike Pilling, who chairs the government’s expert group on air quality, said the public were being misled over claims that Heathrow’s expansion would not cause unlawful and dangerous levels of pollution.

His comments came as it emerged that Ruth Kelly, the transport secretary, has been forced by the scale of the public backlash to postpone her decision on expansion. It was due this summer, but sections of it are now likely to be rewritten.

This weekend the National Trust also came out in opposition to the expansion proposal and to plans by Nats, the air traffic service, to redraw flight routes across the country to ease congestion.

Pilling, who helped to devise the Department for Transport’s rubric for measuring future pollution around Heathrow, said a key recommendation to consider a range of future scenarios was disregarded. He said Kelly’s final conclusion that a third runway would not cause a significant increase in pollution was unreliable.

Kelly had previously pledged that the airport would be expanded only if it did not breach European Union pollution limits. “They mustn’t pull the wool over our eyes,” Pilling said last week. “People are much more sophisticated than that. They [the transport department] need to go back and do some more calculations.”

The government has based its predictions about the impact of expanding Heathrow on a set of optimistic assumptions, including the arrival of cleaner engines. Pilling says that the more pessimistic scenarios were not tested.

“Those residents [who live near the airport] should say, ‘I’m not convinced because you have not looked at all the possible changes that might happen in the future’,” Pilling said.

“They [the department] claim it’s clear that there won’t be pollution [overruns] but they need to spend more time to show there is a very strong chance that this is the case.”

His comments come after a Sunday Times investigation revealed how the transport department and BAA, the airports operator, collaborated to “fix” the environmental figures by selecting the data most likely to get a positive result.

The comments made by Pilling, who is professor of physical chemistry at Leeds University, reinforce concerns raised by the Environment Agency, which has warned that the department’s case is not “sufficiently robust” to conclude that pollution levels will not breach the legal limits set by the EU.

The agency said that more consideration should have been given to variations in traffic emissions, background air quality and climate change.

Kelly is facing protests not just over plans for Heathrow’s expansion but also about her entire aviation policy, with the National Trust warning this weekend that plans to redesign air routes to ease congestion threaten to spoil some of England’s most tranquil areas, including the Chilterns.

Nats has tried to divert some flightpaths from urban to less populated areas, but Tony Burton, the National Trust’s director of stategy and policy, said his organisation opposed the plans because of the impact on some of its properties and the damage to people’s enjoyment of the countryside. One new route out of Luton airport will mean more planes flying over the Chilterns at lower levels.

Under the plans, four new holding stacks are being created to serve Stansted, Luton and London City airports. Some routes are also being changed from Heathrow, with local councils saying that 40,000 more residents will be affected by extra aircraft noise.

The government faces pressure to review both the proposals for a change in air traffic routes and for Heathrow expansion. Serge Lourie, leader of Richmond council and spokesman for the group of councils opposing expansion, said: “The Heathrow consultation has been botched from start to finish. It is an utter disgrace.

“If they are now going to start rewriting the impact assessment, then we deserve a new consultation and not the sham we’ve seen, in which the transport secretary announced in advance that she wanted Heathrow expansion.”

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