The High Court rules today in a new legal battle against expansion at Stansted Airport.
Campaigners argue that the key criteria being applied to decide on possible options for new runway sites in England are "infected by apparent bias".
The Stop Stansted Expansion group (SSE) has asked a judge in London to order the Government-appointed Airports Commission, which was set up last year to address the issue of aviation expansion in the UK, to delay the publication of any options shortlist until the criteria have been looked at again and consulted on.
Mrs Justice Patterson will announce her decision today.
SSE alleges there was apparent bias because Geoff Muirhead, a recently resigned member of the commission, had a conflict of interest.
The claims are contested by the Airports Commission and the Department for Transport.
The judicial review case has been brought by Peter Sanders and Brian Ross, described as:
"Long-standing and key" members of SSE, which wants to contain the development of the Essex airport "within sustainable limits"
The court heard that Manchester Airports Group (MAG), the owner of Stansted since February, has submitted proposals to the commission for a two-runway option at Stansted, and also a four-runway hub airport option which would make Stansted the largest airport in the world.
Aviation expert Mr Muirhead is a former chief executive of MAG. In September, he stepped down as one of the five commissioners appointed by the commission after SSE warned Transport Secretary Patrick McLoughlin they would take legal action if he stayed.
SSE say he retired as MAG's chief executive after 22 years with the group but was then immediately reappointed as "a highly paid ambassador to MAG, a role he continued to fulfil even after he was appointed to the Airports Commission".
Campaigners want the High Court to order the commission, chaired by Sir Howard Davies, "to revisit certain key decisions made by the commission during the time that Mr Muirhead was involved".
The Department for Transport maintains that Mr Muirhead "acted properly" at the commission but stood down to "avoid any perception of a potential conflict of interest".
– DfT spokesman
"Both the department and Sir Howard agree that there is no evidence whatsoever of bias and the Airports Commission is content that decisions taken to date are robust."
An Airports Commission spokesman rejected SSE's claims and described its processes to date as "appropriate and robust".
The spokesman said its selection criteria "were informed by public consultation and incorporate a wide range of environmental, economic and social factors - including local environmental issues such as impacts on landscape and the built heritage".
"The sift criteria demonstrate the openness of the commission's process, and the importance it has placed on an integrated and evidence-based approach to reaching its recommendations."
During proceedings before Mrs Justice Patterson, Paul Stinchcombe QC, for the campaigners, argued that the "fair-minded observer", having considered the facts, "would conclude that there was a real possibility that certain aspects of the process have been compromised, specifically the determination of the sift criteria, such that they need to be redetermined".
He said the shortlisting process "cannot logically begin until that has been done".
The claimants are seeking a declaration that the sift criteria are "infected by apparent bias", and are urging the judge to make an order quashing the decision not to redetermine the criteria.
In written argument before the court, Daniel Kolinsky, for the Airports Commission, argued that it was too late for the criteria adopted on May 3 to be challenged "and significant prejudice to the commission's process and those who have participated in the process by making proposals by reference to the sift criteria will follow".
He submitted that the claim should be dismissed on the basis that Mr Muirhead's resignation on September 20 "sufficiently addressed any concerns that the fair-minded observer would have had as to his participation in the process once MAG had made a proposal for the expansion of Stansted Airport".
Mr Kolinsky said the commission had confirmed on September 20 that Mr Muirhead had stepped down as a commissioner, adding: "That action was taken, on a precautionary basis, to ensure that GM (Mr Muirhead) played no part in evaluating the proposal submitted to the commission on July 19 by his former employers, the Manchester Airports Group (MAG), in respect of the expansion of Stansted Airport, which MAG had acquired with effect from February 28, 2013."