Proposals of five major companies will be considered by the inquiry, which is expected to last 14 weeks.
Oxfordshire councils are supporting a site in Stokingchurch. The county council and South Oxfordshire DC supported this site at a previous inquiry into a proposed site at Tetsworth four years go. The environment secretary agreed with the councils' objections to the Tetsworth site and turned down the proposal.
The inquiry opening next week will consider five sites: three within South Oxfordshire, including one between junctions 6 and 7, and another at junction 8. The other sites are within neighbouring Wycombe district, including the most favoured option at junction 5 being proposed by Shell UK Ltd.