The Campaign to Protect Rural England has voiced "serious concerns" about the Airport Commission's study into airport expansion in the south-east.
The charity told the BBC the final runways report, due out next month, was "bound to be tainted".
The CPRE, which took the last Heathrow expansion scheme to court, says it could do the same again if ministers press ahead with a new runway at either Heathrow or Gatwick.
"Reasonable alternatives have been ignored from the start", the charity's transport campaign manager Ralph Smyth said.
"If the government decides to proceed we are bound to take legal advice as the first step to a challenge in the courts."
The CPRE remains confident of support no matter which expansion option is favoured.
"We were part of a broad alliance of local authorities and charities that in 2010 defeated the last attempt to build a new runway [at Heathrow]," said Smyth.
"We can be sure the alliance this time round will be even bigger."
Transport secretary Patrick McLoughlin revealed that the Commission had considered 52 proposals.
"This included examining whether additional capacity was required and how to make the most of our existing airports and runways," he said.
"The Commission's final report this summer will mark the end of the most comprehensive and transparent process ever initiated by a British government on aviation."
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