Stansted protesters are joined by BA

Campaigners against the expansion of Stansted airport have vowed to fight "tooth and nail" against the building of a new runway at the Essex airport.

They may now form an unlikely alliance with British Airways, which has indicated it will consider legal action to prevent a new runway at Stansted rather than Heathrow.

The legal threat come as it emerged that Transport Secretary Alistair Darling is set to announce in an aviation White Paper later this month that he favours Stansted.

He believes increased capacity at Heathrow would make pollution levels unacceptable.

Norman Mead, chairman of campaign group Stop Stansted Expansion, said that on economic and environmental grounds, "we find the apparent choice of Stansted totally unbelievable".

He said if the Government did now favour Stansted it was "absolutely clear he [Mr Darling] has not listened to a word we have said".

And he added: "We will wait for the White Paper, but people are raring to take action here. People are very angry throughout Essex and Hertfordshire."

He said they had so far deliberately conducted a "bloodless campaign", but threatened that could change.

"People value their quality of life to a greater extent than ever before. Many moved out of inner cities and will defend that quality of life." Fellow campaigner Peter Gowan predicted "big eruptions".

Glen Grindrod, of Direct Action, said the pollution problems that affect Heathrow could also apply at Stansted.

"It's a country airfield which means there's nowhere for the pollution to dissipate - both noise and fumes, " he explained. He also raised serious doubts about whether the region needed another runway, questioning predicted demand.

"If the Government go ahead and develop Stansted because of projected figures, and it doesn't come to fruition, the damage they have done by destroying the area - villages that have been there since William the Conqueror - can't be restored.

"The Government should bite the bullet and put a moratorium on expansion for 10 years."

BA, Europe's biggest airline, said expanding Stansted rather than its own base at Heathrow could be anti-competitive. It fears landing charges would increase at Heathrow to pay for a Stansted expansion, and will hurt its business.

Expanding Stansted first "would undermine Heathrow's position quite significantly", said a BA spokesman. It could force travellers to take flights from other European cities.

The row comes as another pressure group claims the Government has misled the public over alternative proposals for new airports in the region.

The Transport Department consultation-paper on runway capacity in the South-East ignored viable options in the Thames Gateway, says local authority aviation group SASIG.

A new study prepared by aviation economic specialists Berkeley Hanover found the department shortlisted only one new airport proposal in that area, Cliffe in north Kent.

SASIG Chairman Richard Worrall said the consultation "did not give the public the chance to consider whether a brand new airport in an area of low population, where very few people would be affected, might provide the answer".

Edward Lister, the leader of Wandsworth Council, which has been backing legal action against expansion at Heathrow, said: "We believe the Department has misled the public in the way it arrived at its shortlist. It is almost as if Cliffe was set up to fail."

Berkeley Hanover's research also showed that the Transport Department had consistently overstated the economic benefits of Heathrow.