LONDON, July 27 (Reuters) - British transport company FirstGroup said Chief Executive Matthew Gregory planned to step down in September, the day after its biggest shareholder demanded he resign.

New York-based Coast, which owns roughly 15% in FirstGroup, has been opposing the sale of the company's U.S.-based FirstStudent and FirstTransit businesses to Swedish group EQT, arguing the price was too low.

FirstGroup said on Tuesday that chairman David Martin would become interim executive chairman until a permanent replacement could be found. (Reporting by Sarah Young; Editing by Kate Holton)