An environmental storm is brewing over proposals which could lead to further major expansions at Heathrow Airport.

Local residents and green groups are expected to oppose plans which could lead to a third runway and a sixth terminal.

Transport Secretary Ruth Kelly is likely to produce evidence that a third runway would not breach strict noise and environmental limits.

A relaxation of current runway restrictions which would allow for more take-offs and landings is also expected to be proposed by Ms Kelly.

The changes would see the number of flights increasing from 480,000 a year to more than 700,000.

It is thought that a third runway and a sixth terminal could cost as much as £9 billion. Heathrow's fifth terminal - which was subject of a costly and record-breaking four-year public inquiry - finally opens in March 2008.

The Government is bound to face criticism from residents and conservationists at a time when the Gordon Brown administration is keen to show off its green credentials.

Leila Deen, of the climate action group Plane Stupid, said: "You cannot expand airports and stop climate change. The science shows it's just not possible.

"If the Government condones the building of a third runway, Britain will have no chance of reducing emissions in line with Gordon Brown's own targets.

John Stewart, chairman of anti-noise and anti-airport expansion group Hacan, said that the Government's plans could be defeated by the strength of the opposition to them.

This opposition includes some MPs and what Mr Stewart describes as "direct action activists", who between them represented "millions of people".

He went on: "The biggest coalition against airport expansion in UK history has been assembled to fight the Government's proposals.

"Given this level of opposition, the Government could be heading for an historic defeat.

"Never before has any government faced such opposition to its plans to expand an airport.

"For local residents, it is make or break. For environmentalists across the country, Heathrow has become the symbol in the fight against climate change."

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