A campaign group is threatning to mount a legal challenge to the government’s Airports Commission.

Stop Stansted Expansion intends to make an application to the High Court by the end of the month for a judicial review of the process if one of the commissioners, former Manchester Airports Group boss Geoff Muirhead, refuses to resign.

The anti-Stansted expansion group claims of conflict of interest as Muirhead retired from MAG in 2010 but continued to act as an advisor to the company, which now owns Stansted, until January this year – several months after he joined the Aviation Commission, the Sunday Telegraph reported.

MAG submitted proposals to the commission in July suggesting how Stansted could be expanded to a two-runway airport or even a four-runway hub.

The airports company was today expected to announce a major deal with Ryanair, which could see a long-term arrangement over landing charges at Stansted.

Stop Stansted Expansion last month served a formal notice on transport secretary Patrick McLoughlin requiring the removal of Muirhead from the commission on the grounds of “apparent bias”.

The campaign group’s chairman Peter Sanders said at the time that there was “far too much at stake to allow Mr Muirhead’s role as a commissioner to go unchallenged”.

A spokesman for the Department for Transport said: “We will be responding to Stop Stansted Expansion in due course. We cannot comment any further for legal reasons.”