Friends of the Earth is considering further legal action should the secretary of state give the goahead to Heathrow's fifth terminal.
With a decision on the Lord Rogers extension due any day, FOE - the main third-party objector in the public inquiry - looks set to fight on. Unofficial reports suggest the project will be approved.
Aviation campaigner Paul deZylva said FOE would look closely at Stephen Byers' decisions and the restrictions attached to any approval.
The size of the terminal building, the amount of car parking, and whether T5 would serve as a 'Trojan horse' paving the way for a third runway will be factors affecting possible legal action.
HACAN, the local residents group which is against T5, may also join the fight.
The coalition of local authorities formed to fight the proposals has also said it will not rule out legal action. Peter Brown, a spokesman for the 11 authorities involved, said they would scrutinise the decision before deciding their next move.