A government expert has accused ministers of misleading the public over pollution to justify the planned expansion of Heathrow.

Mike Pilling, who chairs the Government's advisory group on air quality, said official predictions were based on optimistic assumptions about cleaner engines and failed to consider the worst case scenario that could see the air over Hounslow severely affected.

"They (ministers) mustn't pull the wool over our eyes," he was reported as saying. "They need to go back and do some more calculations."

His comments came as it emerged the Department for Transport is unlikely to reach a decision on expansion this summer as planned.

Transport secretary Ruth Kelly has promised plans to increase the airport's capacity by nearly 50 per cent will only get the go-ahead if it can be proved there will be no increase in pollution.

John Stewart, chairman of anti-expansion group HACAN, said he didn't expect a decision until October at the earliest.

"The DfT's officical reason is that it's reading every one of the 70,000 responses to consultation, but we know that's never happened," he said.

"The real reason is that they're struggling with the pollution figures."