The government’s Airports Commission faces the threat of legal challenge by campaigners.
The Stop Stansted Expansion group is expected to apply to the High Court for a judicial review of its work, according to the Sunday Times.
The legal action could put the deadlines set by the commission at risk.
An initial report was planned to be delivered to government by the end of the year followed by final recommendations in 2015.
SSE has already succeeded in removing former Manchester Airports Group boss Geoff Muirhead from the commission.
MAG acquired Stansted in January and has proposed a second runway at the airport. It also suggested a four-runway airport.
SSE economic adviser Brian Ross told the newspaper: “For almost a year, Muirhead was allowed to play a pivotal role as the only commissioner with first-hand knowledge and experience of the aviation industry.
“He was therefore in a powerful position to shape the decisions and judgments of the commission to favour the interests of Manchester Airports.”
The commission led by Sir Howard Davies has denied any bias, the Sunday Times reported.